Taiko no Tatsujin (2011) cab, first of all. Ideally I'd get a number of rhythm game cabs, but if I had to choose just one, I'd have to go with Taiko because unlike a lot of arcade rhythm games, there's really no way to entirely replicate the arcade experience at home. You can buy nearly arcade-quality controllers for IIDX, pop'n, SDVX, etc, but even the best home Taiko controller on the market is pretty distant from arcade quality. Plus as far as I know, there's no Taiko HD cabs anywhere outside of Asia due to the required online service not being offered, even at places like Round 1, so that's another reason. For the purposes of this we'll assume I'll be able to get it working somehow, lol.
THE★BishiBashi is definitely another one I'd want. The Bishi Bashi games are some of my absolute favorite multiplayer titles, so I was super bummed that the most recent arcade game never got a home port (aside from a weird F2P mobile thing with a tiny selection of minigames). In the scheme of arcade cabs it's not really too special or unconventional, just 4 huge buttons each for 3 players, it's just a game I think I'd get a ton of use out of that never got a home port.
SCUD Race as well, on that note. I definitely wanted to get a racing game cab in here, and while I'd adore an OutRun 2 SP deluxe cab, I feel compelled to go with something that doesn't have a home release, lol. Daytona 2 is another solid option, but after having played a little bit of both, I do kinda think the higher-speed feel of SCUD Race clicks with me a bit more. But yeah, just give me a cool Sega racing game with a nifty tilting fake vehicle, lol
Picking only 3 is tricky, I'd love to rep some favorite fighting games or lightgun games but if you grab a fightstick or lightgun you're pretty much good to go on those.