What is the best way to use Gtalk on iphone?
Since Google does a pretty good job of making their websites mobile-friendly, navigating to http://talkgadget.google.com/ and logging in should present you with a nice chatting interface.

On the iOS app side, imo seems to be a popular choice, though I can't personally vouch for it (the mobile web interface works fine for me when I'm away from a real computer)