They're too personal of a thing to leave up to other people. Everyone has their own preferences. Some good brands are elixir nanoweb, elixir polyweb, d'addario, d'addario exp martin, martin sp, john pierce, DR, dean markley, and dean markley alchemy. I've tried each of these brands myself and most of them are great. However, not all strings sound equally good on all guitars. Some guitars just sound better with certain strings.
- Art & Lutherie Cedar CW (SOLD! )
- Martin D-16RGT w/ LR Baggs M1 Active Soundhole Pickup
- Seagull 25th Anniversary Flame Maple w/ LR Baggs Micro EQ

Have an acoustic guitar? Don't let your guitar dry out! Click here.
Another thing is gauge. If you like to use lower tunings, a heavier gauge is better or the strings (most notabaly the bottom string) will hit the neck when vibrating and it'll produce an unpleasant buzzing sound. Also heavier strings tend to produce a thicker sound. Just play around with lots of different sets and see what you like best.

Right now I'm using medium-gauge 80/20 Clapton's Choice Martins. They sound great.