In my humble opinion, everyone should own Geometry Wars 2. If you don't already own it, now's the time.
Bionic Commando is a fantastic title too, especially for that price.
I never did understand the fuss about Castlevania SotN though. I bought it not long after it's release on XBLA, based on universal praise, but ended up being disappointed. I suppose I just don't 'get it'. Oh well.