Outside of Pokemon, it was technically DQ1 on Gameboy, but I never left the first room lol. For game I actually completed, that would probably be Mario and Luigi Dream Team. Since then I've beaten every M&L game outside of Partners in Time (which I never played) and was devastated when the series was cancelled last year. I actually haven't really beaten that many RPGs, but I know I have beaten Chrono Trigger, Fire Emblem Fates Birthright, Fire Embem Awakening, Fire Emblem Echoes, Fire Emblem Three Houses, Kingdom Hearts DDD, II, BBS, and III, Final Fantasy I Advance and XII:TZA, Bravely Second, and a couple others I can't quite remember. I also own FF3, FFIV Advance, FFIV DS, FFIV-2 DS, FFV Advance, FFV PC, FFVI SNES, FFVI Advance, FFVI PC, FF7 PS4, OG FF8 PC, FF10/10-2 PS4, base FFXV, FE6, FE7, FE8, Raidiant Historia DX, Takes of Vesperia DX, Octopath Traveler, Dragon Quest I, II, and III Dragon Quest XIS, Pokemon MD2, Pokemon MD3, and Horizon Zero Dawn but I haven't beat any of them and the only one I'm currently playing though I'd DQXI. I also have beaten Zelda Majora's Mask, Links Awakening, Link Between Worlds, Breath of the Wild and Pokemon BW2, BW1, Conquest, HGSS, XY, ORAS, SM, RBY, GSC, and SWSH but I don't think those count lol.
Edit: I've also beaten Castlevania Aria of Sorrow, Dawn of Sorrow, Symphony of the Night, and Portrait of Ruin but also own Circle of the Moon and Harmony of Dissonance. Still trying to get my hands on Order of Eclisia. So rare!
Edit 2: Changed the X to XY for consistency..
Edit 3: Also of note is that I didn't play DQ1 on the actual Game Boy Color or even on an emulator. I played it on some weird flash-game site that had an emulator built-in so you could play Game Boy and Game Boy color games. I remember making it all the way up to Clair on Crystal before the emulator crashed and my save data got corrupted. The site even had some bootlegs, like in infamous Pokemon Diamond! (Not actually Pokemon Diamond like I thought. I was actually a poorly translated bootleg version of some pokemon ripoff game from Japan lol.)