Stay away from Sokoshotels. Terrible. Never again. I have stories one wouldn't believe can be true at a 100 eur/night hotel.
Hiltons in Helsinki are good, albeit all a bit worn down. Fiskis is generally awesome, but with stiff staff who treat old people with respect but have some problems serving younger guests. Strand is in need of renovation. Hilton Helsinki Airport has great staff and overall quite good. All treat Diamonds well in terms of upgrades. Going for HHonours Diamond is now easy, and I recommend it.