Prague has History, stunning buildings, good museums and almost everyone (in the tourist areas) speaks English, but food is not brilliant and nightlife is tailored to Stag/Bucks weekenders, unless you're willing to look quite hard. Good for a weekend, very cheap
Barcelona has History, stunning buildings, very good museums, and amazing food, and a pretty decent nightlife but English is not all that widely spoken. Good for 4-5 days, mid-priced.
Paris has History, unbelievably good food, and amazing buildings and museums, with English pretty widely spoken (but sometimes not willingly) and excellent nightlife. Good for a week. Expensive.
London has History, amazing architecture, a strange dialect of southern English English, some of the best nightlife in Europe, and (yes, really) amazing food (you can find a restaurant from pretty much any country in the world, and the "modern English" style is pretty widespread). Good for a week. Very expensive
Some other places you might want to consider or research: Vienna (and possibly include a visit to Salzburg), Budapest (can be a good add-on to Prague or Vienna), Krakow, Madrid, Venice, Rome.