A top 5 is pretty tricky, it depends what your after, and which night. I like Revolution, thats one of the late license places, its near KFC, across from the Farmers Arms. I also like the Friary and Firkin, it's a church thats been converted into a pub, it's a good place to go and watch football, and is quite cheap if you have a yellow card.
Varsity seems quite good on a Wednesday, i've only been once, but it was only £1.25 a pint. It got quite busy though as lots of people go to Toast on a wednesday night for RnB night, but it empties at about 9 because if you get into Toast before 10 it's free, but the ques are massive.
The Lounge is another ok, studenty, type bar on the way up to Revolution, I think they have an extended license too, but it's not open til 2 every night.
Bentley's has re-opened but I haven't been in since it has, that used to be a good place to stop off and get a fishbowl with my mates before we headed up to Revolution or down to Toast.
The Wagon and Horses on the quay used to be a stop off point on our 18-holer pub crawl we used to do for peoples birthday's, it's a bit away from the town centre, but it's good beer.