It depends of the environment... and a bit the kind of maps. Also you should not forget non analogic pads (some of those using it will say they are using a pad ; and some other that they are using a keyboard, because it's the same kind of moves, but you can hit "keys" faster that on most keyboards)
- Wheel: is probably the best on rally, and (not drifting) island, many players will consider it very difficult on most other enviros. The worst for wheel is stadium.
- Joystick: surely not the best choice for stadium, but nearly as good as wheel on wheel's best enviro, and globally easier than wheel for all others. It's better than gamepad when a very good analogic feeling is useful, probably more difficult than gamepad one maps with many consecutive short curves. It's probably the best controler if you really take time to learn it and succeed to master it.
- Gamepad(using analogic stick): is the most adaptive one, can be nearly as good as the best one for each enviro. It's also the best for lazy players : it's clearly the less exhausting one to use ^^
- Keyboard(or numeric pad): just the reverse of the wheel, it's better on stadium and very very difficult on rally. Strangely it can be very efficient on coast.
Note that you will find some exceptions in all cases, anyway, seeing the total number of players using each, and how many of are the very top of best players, except if you play stadium only or rally only, the joystick is probably the one with the best ratio (but it can also just mean that several very good players have chosen the joystick).
It would be very interesting to see what controllers are used by the very best players of each enviro : for example i'm pretty sure that for rally more than 50% of the very very best players are using wheel or joystick (while wheel+joystick players are probably less than 1% of all tm players)... but of course don't think that wheel or joystick are that easy on rally: it's wrong ! If you are a very good rally driver and have a very good level on both gamepad and wheel, then using wheel on rally you will probably be able to drive just a little faster.
So...
- if you want the best efficiency/laziness ratio, use the pad
- if you don't play stadium only, are willing to work hard to learn it, trying the joystick is a nice option.
- if you want the fun to drive a car using a wheel... try a wheel
(and in any case, don't buy a 1st price wheel or jostick ! better choose a gamepad than a bad wheel or joystick...)