I still have a ton of the library to explore, but my top recommendation would be Laser Lords. It's a dense adventure/RPG hybrid that feels like computer RPGs of the late 80s/early 90s. The conversation system is surprisingly complex and really interesting; less of a conversation tree, more of a system that lets you pick up on individual points you're interested in exploring further. Instead of items, you collect information by adding those same dialogue branch points to your "inventory" where you can later ask other people about them. It's fascinating and it feels great. It takes place within a surprisingly huge open world across a bunch of planets, so it's easy to get lost, but the experience of getting lost while chasing up leads and making it further is really good. As far as I know, it's playable in MAME.
A recent CD-i homebrew from last year, Nobelia, is another one of my top picks. It's basically Zelda meets Bomberman. Each screen is its own standalone puzzle which feel a lot like Zelda-style puzzles incorporating Bomberman-style mechanics, and like Zelda it doles out new skills periodically that unblock you in solving other puzzles. It's well-designed and very, very fun. It's available as a download from Itch or on CD. https://twburn.itch.io/nobelia