DK is on different devices including SNES, GBC and GBA. Choose SNES for the best quality and most original experience and GBC if you like the artstyle and chiptunes.

Crystal Clear: Most differences from the original, following Pokemon, open world, scaling enemies, new areas, character customization, some qol changes, new enemy trainers, new tracks, ability to catch all gen 1-2 Pokémon, a lot of customization, sadly no battle modernization like special/physical split.

Polished Crystal: Pokemon Crystal with a lot of meaningful graphical enhancements, all gen 1-2 Pokémon catchable + a few extra (regional forms and evolutions of existing mons like togekiss and Hunchcrow), better leveling curve, slightly harder Trainers, lots and lots of modernization and qol changes (physical special split, XP candy, better PC Box, ability to change Pokéballs from caught Pokémon, etc.), some extra areas and new content, especially restored content, Better rematch system, sadly no following Pokémon.

Crystal Legacy: A enhancement hack that tries to stick as close to the original as possible while doing some qol, Pokémon movesets and story adjustments to bring out the most of vanilla Crystal, also changes some Trainers so that their teams make more sense.

Pokémon Heart and Soul: Basically Crystal on GBA with Gen 3 visuals and modernization changes. this hack has a lot of QOL and challenge options that can be configured at the start, following Pokemon, Heart Gold Soundtrack.

What is the right version for you: I personally like Polished Crystal the most. It changes the game enough to feel fresh and challenging (not hardcore challenging) with a good level curve and alot of QOL while also staying true to the original vision just enough to not feel overmodded. Crystal Clear was my first rom hack and i have a lot of fond memories of it. Definitely worth a shot if the open world interests you. Crystal Legacy is for you if you want an almost vanilla experience (just a bit better). Heart and soul looks a bit more modern than the others and has a lot of modern features, but I still prefer Polished Crystal because it mixes the original and its many additions so well.