Uh that's a good question! And for some games, "beaten" is hard to define...
From the top of my head, counting only those with Mario in the title:
10 Sidescrollers:
SMB 1-3, Lost Levels (in SMAS), SML, SML2, SMW, SMW2 (Yoshi's Island), NSMB, NSMBWii
5 3D games:
SM64, Sunshine, Galaxy 1+2, SM3DL
5 RPG's:
SMRPG, Paper Mario, TTYD, Mario&Luigi 2-3 (not quite beaten M&L 1 and SPM)
6 Karts (not quite beaten MK64)
Double Dash
1 Sports:
Mario Tennis
= 27, my favourites in each category are bolded.