Benidorm would be my venue of choice, done two there and its a great weekend.

Some of the popular ones now are mainly in Eastern Europe .
Heres a few suggestions

Prague
Krakow
Bratislava
Tallinn (estonia)
Ljublijana (more expensive as this slovenia and euroland )

Also possibly Riga and budapest.

We have been to many of these destinations although not on stag do's but there is plenty of that going on . All are easily accessible with cheap flights from the likes of ryanair, Easyjet flybe but none are as cheap as they probably once were for drinking but certainly all part from Ljublijana are much cheaper on accomodation .

I havnt been to bratislava or riga but out of the rest my top pick would go to Krakow and closely followed by Prague and Tallinn .