On/Meets is the newest, it's very similar to the M!x which preceded it except it has bluetooth to connect to a phone app (and you can send custom/modded content from your computer using tools made by the community). Also noticeably laggy but not enough to be a dealbreaker.
The On/Meets/Mix are fairly simple, it's focused entirely around seeing what character you end up with by breeding genetics. Most color tamagotchis before that had goals around raising all the possible characters (with some strategy around type of care or skill points), but with gene blending and infinite combinations they took that out.
The P's has the most content, characters are based on earning skill points rather than just care misses, and there are "deco pierce" amiibo-like attachments which give you either costumes or outright let you evolve into DLC characters. Every deco-pierce has been converted into digital files that you can send through an IRDA pc dongle or a phone with IR. Like the ID/IDL/Meets you can also make your own wallpapers and outfits using tools made by the community. The P's also has a large range of fan-made DLC characters. All of that requires you to have some kind of IR sending device, but even without it I think the P's has the most content and depth of any color tamagotchi.
The 4U came after P's, it's dumbed down a bit and is heavily reliant on having an NFC android phone compatible with an app. A bit more experimental rather than a clear successor to the P's
Imo:>Color > ID > ID L > Ps
was a natural progression of improving and adding content to a traditional tamagotchi>4U > 4U+
was sideways/backwards, simplifying things to target a different demographic>Mix > Meets
was sideways yet again, this time focusing entirely on the new genetic breeding mechanic. No school/jobs/etc, just lots of locations to meet characters you can breed with