I can't say I have a list of "Must have" games, because I don't. But I do see this machine as an opportunity to install controls that I don't have on the other 2 cabinets. My main cabinet is horizontal, SF2 layout +4 way, + Q*Bert, + trackball and spinner. It is a good universal setup that supports MOST titles. Then I have a 4-way cocktail with one button per player. It has a short gamelist, but that includes a lot of good games.
I just got to thinking that I would probably rather have a spinner rather than cocktail mode Centipede, and next thought was that I really shouldn't let my rotary sticks go to waste (I have 3 Wico rotaries, all in bad shape, but they will combine to make one really good one). and heck, I might as well add Tron support if I am going to have a vertical setup with a spinner, so that is how I came to that layout.
I'll still be able to support cocktail mode on standard 8-way games and even spinner games (although one player would get the trackball). But I would be adding 2 player support in non-cocktail games, the rotary games (single player support).
I might still change up the design some more. I'll likely have to add a second encoder to get everything as SEPARATE inputs. Which would be good, as that would allow for 4-player Gauntlet, and two player Smash TV. Note, those are both horizontal games, but they can be two player simulatneous vertical if you just map the controls properly.
The only thing I am missing is an analog stick, but I can't think of any vertical analog games worth playing. I have considered adding a gas and brake pedal on the side with the spinner.