Depends on your playing style.
The heavier the string, the harder it is to do bends.
The lighter the string, the more 'twangy' it sounds.

For a good mix of both, try Dean Markley custom lights.
9, 11, 16, 26, 36, 46
Light on the top strings, heavier on the bottom.

You'll have to adjust your action and intonation when you change string sizes too... although I'd recommend doing that if you're getting a buzz on one fret anyways. You might also check to make sure that the actual fret itself is properly seated. Try to figure out where the buzz is coming from if you can.. ie, at the neck, at the headstock, at the bridge etc..