First, how much money do you want to spend? If you want to keep it on the low side, JnJ, Lapco and Dye all make good low-mid range barrels. Personally, I'd go with a Dye Excel or Dye aluminum. You can usually get them for about 50 bucks or less.
If you usually use the same type of paint when you play (for example, you always go to Three Rivers PB and buy Blaze), try the paint in the barrel before you buy it. Different paints have slightly different sizes. It's no good getting a medium bore barrel if you're shooting large paint.
You should just be able to blow the ball through the barrel without it falling straight through. If you can't blow it through, the barrel is probably too small and paint will blow up in your barrel more often.