There are waaay too many bars to choose from. I tend to separate them into two categories: Freshman bars, and townie/21+ bars. If you are going so you can have a crazy experience surrounded by many girls, loud music, and excitement, go to a freshman bar. If you want a more relaxed, good bar experience where you don't have to elbow your way to the bar to wait 15 minutes for a drink, go to a townie bar.

Some freshman bars: Bourbon Street, Buddha Bar, The Loft (which is also a dance club), Barcode, and many others.

Some townie bars: The Globe, Trapeze, Lans Downe Road (sometimes gets pretty packed).

I will try to think of others.