Spoiler:

Currently my favorite one is the Drivable Motorcycle Mod for Fallout 4. It's the only vehicle mod I've been able to use without the game crashing within 5-10 minutes of using it. Plus the motorcycle is customizable and your companion can ride with you on the back if you choose. There's also the fact that you can increase and decrease speed with either hotkeying the speed options or pick a default speed. Overall I think it's pretty solid.

I liked the Driveable Vertibird mod for Fallout New Vegas. I remember strafing Goodsprings and the highway. Take THAT giant ants.

I like the Mobile Truck Base by tdx73 for Fallout New Vegas although you cant manually drive it the storage and all round convenience of it make it a must and its a great alternative to fast travel there is also a new release for Fallout 4 that has the same features APC Home on the Move  by bostek is also a very well made mod.

My favorite is probably XRE Vertibird. I feel pretty overpowered in it.

There was this one drivable vehicles mod on FNV i used to use it the behemoth addon with that mod was pretty neat

I play with XRE cars but I never drive them.....the physics are too brutal. The engine clearly isn't made for having vehicles that actually drive. I play with the mod for the ambiance of having the service stations and cars. My favorite 'vehicle mode' when it works, is a hoverboard.

In my head cannon, my Courier has a hoverbike, and this basically gives all the mechanical benefits: it goes on water, it climbs and gently falls down hills of any steepness, and it's a very very smooth ride.

Vertibird Player Home by Hopper31 gives you a unique Vertibird home complete with an acceptable amount of safe storage, work & reloading benches, a co-pilot as well as the option to pilot the bird yourself in either first or third person! You can shoot a variety of weaponry from it in manual flight including guns, missiles and mini-nukes. The icing on the cake, though? It comes with a few custom skins you can choose from in the files section! Even includes a skin used for Vertibirds in the upcoming Frontier mod.
If you don't feel like flying it yourself, you can set a destination in the navigational computer and take a load off inside or even sleep. You're given a pair of binoculars which you can use to either summon the bird to your location or command it to do bombing runs ala Enclave Commander.

http://www.nexusmods.com/newvegas/mods/60923/?

Other than that, Fallout: Who Vegas adds a full-fledged player piloted TARDIS to the game along with a series of brilliantly written quests, weapons, armors, worldspaces, companions, planets, enemies, etc. The TARDIS itself is really more of a glorified fast travel alternative/player home hybrid than a vehicle but ehh what can you do. It's still pretty neat.

http://www.nexusmods.com/newvegas/mods/50132/?

The drivable vertibird is a must have! I cannot express how much fun I had strafe running legion troops X).