Frontier has best vehicles so far.
Some alternatives are XRE mods (2012):
XRE beta1_35 showcase - Flyable Vertibird
XRE - CARS v0_86
Or something ancient like this (2009):
Driveable Chimera Tank
J3Xified Driveable Motorcycle
I remember discussions from 2010, where most of the community didn't want vehicles. Also making a vehicle mod is incredibly difficult task.
If you want vehicles, bite the bullet and get the Frontier.