What is the best cruise for a group of about 8 young adults?

humbug12 asked:

I’m going on a cruise with some friends who are all ~23. We’re looking to do a 3-4 night cruise on a modest budget (less than $1000 for the whole trip-including airfare). I was wondering what destinations you would suggest? Cruise lines? Specific ships? It’s my first time, and I just don’t know where to start.