I stayed at Center Hotels Plaza on my first night in Reykjavik, and Center Hotels Klopp on my final night.
If you're a light sleeper, you definitely want this one (Plaza) and not Klopp. I slept well here.
If you like to be close to the nightlife, either hotel is a short walk from the action, although Klopp is a little closer.
If you want a large hotel room... maybe look elsewhere. I couldn't believe how small the hotel room was when I arrived! However, for a solo traveller, it was fine enough.