Not necessarily success or numbers vises but MUSIC wise.
For me it's undoubtedly Fifty Fifty! Their debut ep Love Tune is immaculate. A kpop gem really. Special mention to Meovv. Their debut song was cool, trendy, hip. For a debut song it was ok, it wasn't great but it was a nice introduction to the group's sound.
Kiii Kiii is major blop for me bordering flop. the debut song was a cringy flop. h2h is just ... bland and non remarkable really even thought there are intersting and potentially promising elements in their debut tracks. For BM, well the only song i could listen from them was Drip but their debut tracks(?) were both terrible. A flop to me again.
For Illit, there was alot of potential (music wise) but very poor and mediocre execution. LGS was decent tho, if not fine. But again, the vocal execution specially isn't really interesting if not mediocre. their second ep had interesting songs but again bad execution. I liked Tick Tack the best (and iykyk had some potential). Cherish was a decent b side at best.
So yes, Fifty Fifty DEFINITLY stood out and came out victorious. No offense and i wish those groups well but seeing I do me and The Chase rise on melon while Love tune tanked kinda hurts. I wish them success really. Their songs are just way too good. From Higher to Tell Me to Cupid to Starry night and Gravity, 50504's discography is a smooth GEM.
This and Gravity are my favorite songs from 5th gen 
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epe-puddle :.
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