Which platform is best for playing racing games. Playstation portable, iPod or Nintendo?
The PSP is probably your best platform for good racing games. Mario Kart is cool, but when you say Racing, I think PSP. The library is larger, the games are realistic, the physics are good and you have better controls for this type of game.
I would definitely suggest the PlayStation portable as these games are more "real life". iPod, is an MP3 player, and not designed specifically for games. Nintendo DS, games are more cartoony, so if you like games that you use to play on a Super Nintendo or Nintendo 64 or Wii, then get the DS.
But real life games like Gran Turismo, I would go with the PSP
I assume by "Nintendo" you mean the DS.
Of the three you listed realize that the PSP by far has better graphics and it has a better collection of games as far as realistic stuff goes. For example, check out this video to see Gran Turismo running on the PSP: http://www.youtube.com/watch?v=QypIvuYDPn0 (skip in a bit for gameplay)
The DS has the dual screens and touch interface which is cool for some games but I don't think is very suitable for racing. Also they tend to lean towards Mario Kart style games which are definitely not my thing.
The iPod you have to realize has no physical controls which is usually a pain for most games that aren't puzzles. Most racing games on the iPod Touch rely on tilting it back and forth at the expense of things like handbreak drifts. The iPod is mostly a device for listening to music, and isn't set up with gaming in mind.