Wow, what a broad question! There are soooooo many choices!!
Absolutely the very best advice I can give anyone asking a general question like this is to find a good local Travel Agent who is a CLIA Accredited Cruise Counselor (look for the agent's diploma on the wall, not just an agency sticker). Ask your friends and co-workers for a recommendation. The service is entirely free to you. ACC's are trained to find the very best cruise for you, after learning your personal situation and preferences, and work hard to get you the very best rate.
Once you have some idea of the possibilities and what you want to do, then maybe we can make some specific recommendations on this board.
When we considered this question and did our research, the answer we came up with was to cruise on Princess to the Baltic and St. Petersburg. It was a great cruise. Check out our trip report and pictures at http://www.bully4.us/baltic.html
But -- your answer may be entirely different, depending on what YOUR expectations are and what YOU want to do!