The answer to your question really depends on a number of factors:
- Do you approach it as a "game" or a "sim"? I.e. what level of realism do you want? For a realistic experience, you just can't beat a wheel and pedals.
- What kind of controller do you use? A cheap plastic wheel with bungie cord springs isn't going to give you the same level of enjoyment and realism as a high-quality wheel. However, you can pay a lot of money for dedicated controllers that will end up seeing relatively little use.
- Does the game support multiple analog inputs, and/or allow you to map the inputs according to your play style? Regardless of controller, in many games it is important to be able to feather the throttle, etc. I really hate the games that only give you a couple of preset controller mappings, since they almost never match what I want.
For myself, I have used the following:
- Home-built wheel/pedals (can't use it anymore because my computer no longer has a game port)
- Act Labs Force RS wheel/pedals/shifter (with "pro" pedals)
- Multiple gamepads and console controllers
Of these, I still use the Act Labs whenever I want to do "serious" racing but the gamepad is sufficient for arcade racers. I've played all the way through NFS: Underground with the wheel and a gamepad, and wouldn't say it was significantly better one way or the other.
Background/disclaimer: for a number of years I was deeply involved in the racing simulation scene, including an extended stint as the webmaster of Sim Racing Connection where I also maintained the track/addon lists for GPL, DTR and SCGT. I've played just about every racing sim/game ever made, all the way from Grand Prix Legends to Mario Kart. :)