I know I'll be getting gruff from veterans of the game like Kilrahi and Roadie, but here it goes.
This is my rendition of the TMBO Tier chart and why they fall under that rank. The best place to start will be at the top going from S Rank to F Rank with S being the best and F being the worst. Again this is just my opinion based off of the years I have played TMBO.
S. Warthog:
There are many reasons he is at the top of the list. He has the fastest recharge time for a special weapon in the game. Does horrific damage with his special and his fireballs home in on targets well, doing insane damage at the same time. (in just a few seconds a vehicle can go from green to yellow roughly 60 dmg, they also make no noise when hitting enemy targets). To top it all off he drifts really well and can move quickly for a vehicle of his size. Warthog is God in this game (Sorry Kil).
A. Manslaughter:
The 2nd best vehicle in the game in my opinion. He is heavily armored and has a good drift when doing turns. The Rock Cluster special is what puts him near the top. A rear fired rock cluster on an unsuspecting vehicle directly behind him can knock them from green to red to even dead. It is just as effective firing the cargo of rocks forward, some clustering on one vehicle doing about 40 damage or more. If that were not enough he even has a sheild glitch in which the force field is not readily visible from vehicles from afar.
A. Axel:
This vehicle has some of the best attack options offensively as well as defensively. His Shockwave does 20 damage roughly per vehicle in it's range. It has a wide area of effect and hits enemies behind walls. He can dodge just about anything really easily, it's close to impossible to get a missile lock on him. Also, his secondary doubles as a shield (increases his armor class to Warthog like levels). The Rollerball attack does a respectable amount of damage to any one vehicle in his way dealing about 45 dmg depending on armor of the opponent on the receiving end. If he had his ability to rollerball multiple targets with one special from a game I choose not to discuss, then I would be inclined to give him a God status of S.
A-. Darkside:
This vehicle is amazing as you can be creative in many ways in using her Special Weapon which makes her fun to use (far better than her predicesor in the new TM). Rear Ramming or even T-Sliding works wonders to vehicles and you can even attain ramming bonuses. Her special is lag proof, giving it priority when playing online. Her only drawback is speed, but then her Ram can double as a nitro boost. Her special recharges fast too. There are few vehicles that can topple another vehicles with just a few hits, and Darkside is one of them.
B+.Crazy 8:
Crazy 8 is one of my favorites. When the Tazer connects it does fair damage in blue form, but in violet form its damage increases at least two fold and if unchecked can deal about 60 damage in one sequence. This is only possible if your opponent does not sheild otherwise the Tazer Attack is wasted. If enough specials are saved however, the Tazer Attack can be spammed and nothing short of taking out Crazy 8 would stop it. The fact his special can go through walls and ceilings is what makes Crazy 8 an above average vehicle on this tier list.
B. Shadow:
Shadow has some great tools in her arsenal. One is kinda useless the other however is her gem, the Shadow Bomb. The reason being is that the Shadow Bomb can hit multiple targets and does roughly upwards of 45 damage per hit per vehicle. If the purple lights in the shadow align together the damage increases. Like several other special weapons in the game, the special can be spammed making her somewhat a controversial vehicle to play as. Shadow Bombs can hit enemy vehicles behind walls increasing her vehicles worth overall. Her secondary can be dodged completely if done correctly, sort of lowering her tier wise. She has slightly above averarge armor with a respectable handling and dodging capability.
B. Junkyard Dog:
Billy Ray Stillwell has a very decent vehicle to use in the contest in Junkyard Dog. His special weapon consists of a launchable, explosive, wrecking ball that deals roughly 30 damage per hit. You can hit with it multiple times if you save plenty of them in your weapon inventory. He moves a bit on the slow side but drifts well. Unfortunately, he is usually the first vehicle everone goes after as he is quite the threat in a battle. Overall, it's a great vehicle having a great balance of power and armor.
B-. Minion:
This vehicle has just about the same special as Warthog with one big difference a very slow recharge time. Minion is just not in the same league as Warthog as Warthog for one can outrun him and Minion is a much bigger target. He does however have the best armor in the game and takes alot longer to destroy. He moves fast enough, has a great drift. On a bad note, he also has some terrible control, and is somewhat cumbersome to drive. Again, the reason he is a "B" class vehicle is mainly his Special Weapon Power.
C+. Yellowjacket:
Another vehicle that has many attack options making it a cut above average. His special can be used as a projectile as I've used this option many times and on average a player can get about 20 damage when going this route. The better payoff however is ramming a vehicle with the spikes out, usually a T-Bone charge will get the maximum damage. If used properly, Yellowjacket can easily take a vehicle from green to yellow in a few moments. If he "pops" a vehicle it can get them to red sometimes. His regeneration time is about average for his Special Weapon, so it's worthwhile vehicle to use overall. Speed, dodging and maneuvering all all great for him, just don't use him for maps like Suburbs unless you are really looking for a challenge.
C. Roadkill:
Many have called him a gentlemans pick, and I agree. Roadkill has a special weapon that takes skill to pull off. Unfortunately, it can be heard when charged, and that definately takes away from the element of suprise when attempting to attck with the Missile Barrage (which is why he's a "C" rank vehicle vs. a "B" rank). Fully charged, they can take a fair chunk of health from an opponent. What saves him from being lower on my tier list is Roadkill has great maneuverability, having fair armor and is a great vehicle to use for dodging homing missiles and the like.
C-. Spectre:
Spectre has been known as perhaps the cheapest vehicle in the game. She does have some benefits, although not quite enough to push her into the higher leagues. Her Ghost Missile has the best homing capability of the game, but it moves slow and can be seen and heard. The recharge time is only fair and it can be dodged or outran. Saving them is probably the only good strategy, as odds are once an opponent has used a sheild they probably won't have another. The fact that the shield is the same color as her vehicle makes it difficult to tell if she is sheilded or not.
D. Outlaw:
Outlaw is a bit of a risk to use. I have to say without the Blue Missiles of Outlaw, it would be like sending a mongoloid into a fight with Ken from Street Fighter. The Blue Missiles are Outlaw's Bread and Butter dealing about 8 dmg per Blue Missile. They add up and if a vehicle shields or dodges some of the attack, you can at least run away with about 24 dmg on average. Unfortunately this is is where the "D" rank comes in, his special can be completely avoided by driving a certain way to avoid the gun turret. So in essence, this makes him all but useless. On the plus side he does have great armor and drifts pretty good.
D. Sweettooth:
As the star of the franchise, you would expect Needles Kane would have a wicked vehicle to use, unfortunatley this is not the case. His special much like Outlaws can be dodged and altoghter avoided sometimes by just hiding behind a corner or sidestepping it. The recharge time for his special is bad, and does not drift all that well like some of the larger vehicles. An effective strategy is to fire a freeze hope it lands (you are screwed if this is dodged) then fire away taking about 45 damage total. Also, there is a chance the missiles will crash into the ground, so being aware of the angle you fire them is a must.
D-. Mr. Grimm:
Sadly, Mr. Grimm is really low in my tier list. On the plus side his Scythes can be fired continually linking them one after another. The weapon works really well when used in combination wth the freeze making this vehicle a very real threat. His Scythe on the average does about 50 damage (guaranteed when they count) . There are some problems with the scythes such as they have a tendency to move away from your target the further they go out or hitting your opponents ghost not doing any damage. The thing really keeping Grimm in the lower ranks is his low armor and a Special weapon that is not all that dependable.
F. Brimstone:
This is definately a skill vehicle as there are so many drawbacks for it (mainly in the Sinner Special). The special weapon has poor homing capability and tends to miss often(using his drift/slide helps a little but not on bumpy terrain). This is even when you try to lead your shots. Another sad thing about the special is from time to time a "Diet Brim" is fired doing little damage (about 5-10 dmg vs. 25 dmg) . No other vehicle shares this unfortunate characteristic. The weapon can also be neutralized by a rival player by holding down the shoulder buttons at the right time and has a slower recharge time. The vehicle itself however is decent having great drift and maneuvering capabilities along with having excepional dodging.