We went to Mosaiik for dinner on our first night in Kuressaare and weren't dissapointed. The restaurant was empty when we got there, but to be fair we visited during January when the whole city was pretty much deserted. The service was really good, atmosphere nice, good music and the food was great.
My boyfriend said the beef tartare was the best he's ever had (Pegasus in Tallinn definately has better, but it was top quality indeed). Duck and pike perch were delicious, if I'd have to complain about anything then just the sidedishes were a little plain. Had Mississippi cake for dessert which was also delicious. Oh and the gin&tonic was topnotch.
The next evening we also visited Retro cafe which was ranked #1 in Kuressaare (Mosaiik #3) and I'd definately recommend Mosaiik over Retro.