I would also recommend staying in downtown Helsinki -- drive to the Messukeskus is very quick.
I think
Palace Hotel is very nice, centrally located, and has the best views over the harbor.
Klaus K Hotel is a new boutique hotel in a central location. I haven't seen their rooms, but supposedly they are nice.
Also Scandic Simonkentta and Radisson SAS Plaza are recommendable and centrally located. Radisson SAS Royal is ok too. Hilton Strand is nice (and has nice views) but it is about 10-15 min walk from the most central area.
Kamp is a fine hotel, but doesn't have much style in my opinion (it is a nice building outside, but I find the interior a bit fake for some reason... supposedly grand old style, but built with cheap materials). Hopefully their restaurant, which is now under new management, is better now. I had too many bad (business) lunches at the old one...