We've been to Helsinki, Tallinn, Riga and Vilnius. Contrary to some of the opinions above, we found Helsinki to be a wonderful city, worth another visit IMO.
Same with Riga.

Tallinn also was interesting.

Vilnius not so much. If you're only choosing between these cities, I'd skip Vilnius.

So, in answer to your question, I'd go with Helsinki, Tallinn, and Riga.