@jfrmilner Thanks for the kind praise. My answer about multiplayer games is going to come from the arcade game options, as my build obviously has that emphasis. There is another thread here somewhere that lists all of the 4-player games (4-player MAME lists can also be found online). But I have far far less than a full ROMset loaded on my system--I load games as I learn about them. That said, it is hard to beat a good game of 4-player Gauntlet and Gauntlet 2. Try Xenophobe. Then, there are the fighter games like X-men, or the Simpsons and TMNT. Battletoads has 3 player capability, Rampage is fun. Then there is a handful of sports titles. Stone Ball is super fun! NBA Jam, and a few racing games like Super Sprint.
My build looks like a 4-player cabinet, but remember, many classic arcade games have the CRT oriented vertically, so for those, even though they are not 4-player games, I play them from the vertical sides of the cabinet (players 3 and 4) to get the best, full-screen experience.