5th Ave is OK every so often, but I went through a stage of going there every week and it just became boring (the same music every time, same people, etc). However, if your just out to pull and drink cheap alcohol, its perfect as its full of horny young boys and girls, and during the week there is almost always some sort of drinks deal (£1 a vodka & red bull...score!).
However, I think the best thing to do in Manchester is simply go everywhere and try everything out (even if its somewhere with a bad rep - you may surprise yourself!). There is sooo much to do. My advice is simply thoroughly examine all the clubs and bars along Canal St, Oxford Rd and Deansgate Locks (as well as 100s of others). One thing is guaranteed: you'll never get bored.