>>324467241
Forgot to mention Tea for God. The gunplay is eh but it's use of non-eucledian worlds is unique and very well suited for VR.

>>324467478
I didn't enjoy the gunplay in HL:A personally, I consider it better as a horror/ atmospheric shooter but if you have it and enjoyed it then no harm in trying the custom campaigns. 
I forgot about the doom3 mod but I've heard good things. Serious sam may be fun in co-op too from what I've heard.