I've only ever done the open ladder, so I don't have any max speed vehicles. I use two for snaking and one for flying:
Snaking
Highway Gantlet-V2
- Body: Dread Hammer
- Cockpit: Sonic Soldier
- Booster: Thunderbolt -V2
Highway Condor-V2
- Body: Brave Eagle
- Cockpit: Sonic Soldier
- Booster: Thunderbolt -V2
Flying
Combat Dragon
- Body: Speedy Dragon
- Cockpit: Combat Cannon
- Booster: Sunrise 140
Aside from these custom vehicles I also use Fat Shark a lot. These may not be the best per se, but they're definitely competitive enough.
You can refer to Mr. Blarney excellent
Custom Machine Guide on GameFAQs for how to unlock the parts for these. Basically, just beat some cups on expert (we want to anyway, for master difficulty) and some story missions on hard (we want to anyway, for very hard difficulty).