hi, I have an unlocked iphone 3g with ios 4.1 and baseband 6.15.0... I have the following questions

1. Which is the best ios for iphone 3g?
2. how to downgrade to that version without locking the iphone again?

You can never "lock" the phone again the 6.15 baseband is permanant

best iOS for 3G - either iOS 3.1.3 or iOS 4.2.1

It's possible that the combination of iOS 3.1.3 and baseband 6.1.5 won't work (I've not heard of anyone testing it)

All your questions could have been answered with a quick search - would have saved you the effort of posting and waiting for a moderator to find your post and move it to the right section
Closing this thread as these questions are duplicate.

See http://www.hackint0sh.org/f204/196391.htm for details on how to restore your phone with the 6.15 baseband.