Here's the ones I spend most time on:
Gigawing Mad shmup with mad unpronounceable scores. Great fun and the rechargeable shield makes for very skilful play.
Gunbird 2 My favourite vertical DC shmup. Great design and the different characters play differently so provide loads of replay. The coin chaining mechanic adds to the score chasing too.
Pro Pinball Trilogy This looks simply stunning via VGA. Really deep pinball play on 3 detailed tables. Very slick.
PenPen Ok so its a bit light on content but I really like the mix up of styles and the courses get really good later on in the game.
Virtua Tennis (1&2) Brilliant arcade tennis. I use this as my June office tourney game played through a projector with the arcade sticks. Epic - accessible and deep for the experts.
Mr Driller Addictive as anything you will find. Beautifully presented too.
Cannon Spike Short but wildly addictive and so replayable. Feels indulgent and luxurious as there's loads of fan service.
Caesers Palace 2000 Niche product but does what it does so very well. Blackjack and video poker - the only skill games you can get in a casino are presented very well with plenty of options and a coaching mode.
Outtrigger I'm not an FPS fan but I love this. Comes to life with the keyboard and mouse.
Bangai-O More genius from Treasure. Lovely looks and the risk/reward mechanic is great. Nutty Japanese presentation is the icing on the cake.
Rez DC is the home of Rez - gorgeous via VGA.
Sega Rally 2 Stutters a bit until you use the VGA box. Horribly addictive and that great slidey control.
Daytona 2001 My favourite racing game anywhere. Just has the Sega magic. I love the look, the sounds, the tracks and that trademark Sega blue, sparkling sky.
Typing of the Dead Sheer Sega genius. Noone else would have dreamt this up - I miss Sega. I use this as the Halloween office tourney event - a brilliant 2 player versus mode that is really well thought out.
Hydro Thunder Big, daft boat racing with dodgy, cheesy presentation (pity its 50Hz only in PAL) but I love it.
4 wheel Thunder Lovely OTT racing game with some enjoyable battle mode style 2 player games.
Street Fighter (Alpha 3 , III 3rd strike) Gorgeous fighters with a bewildering array of options I will never understand but I love the fact that I own these games.