The Gameboy Advance ones are fun. Especially Aria of Sorrow, which got a sequel on Nintendo DS called Dawn of Sorrow. Dawn of Sorrow is one of my favorite games of any series!

Those two games are unique in that the main character, Soma Cruz, can absorb souls from enemies and use them as magic spells. Mostly for attacking, but also for defense or utility. It adds so much variety. Some souls are like progression items and you're guaranteed to get them at the right time, but most are random drops. Getting more of the same soul (up to 9 of them) makes the effect more powerful, so it leads to farming the same monsters over and over if you like a certain spell and want to level it up.

This mechanic is so fun that when Castlevania creator Koji Igarashi left Konami, he produced Bloodstained: Ritual of the Night (not retro) with the exact same mechanic. His own favorite games in the series are SotN, and Aria of Sorrow. Once you're done having fun with retro Castlevania games, you must try Bloodstained: Ritual of the Night! It's so good!

Bloodstained: RotN even has a built-in randomizer. I'm not sure if you're familiar with "rando" games, but lots of old-school games have rando versions: Zelda games, Final Fantasy games, SotN, etc. They use logic to randomize the order you get key items, but still allows the game to be completed. In games like SotN, they even randomize which items each enemy drops, so you might get something really powerful from an early enemy.

There are Twitch channels where competitors race against each other in real-time using copies of the ROM created with the same randomizer seed, so they're all playing the same exact game, but no one knows where anything is. It's fun and amazing to watch very skilled players play these games where no one knows exactly how to finish it, but they are speed-running it.

SotN is my all-time favorite game, and I've played it as rando many times, and the built-in randomizer in RotN is also tons of fun.

The other Nintendo DS games are also great: Portrait of Ruin and Order of Ecclesia. All three DS games have such beautiful pixel art.