In my personal opinion, 12th gen through 15th are all pretty good.
Good/Memorable members from each gen:
12th - Tano, Tomu, Yukarun, Hirari, Miyupon, Juri, and (formerly) Karen and Erena.
13th - Saho, Umetan, Ryoka, Yuiri, Mogi, and Ayanan.
14th - Naachan, Mako, Miki, and (formerly) Nakki.
15th - Manami, Megu, Seichan, Yuami, Miyabi, Miion, Naanya, Tatsumaki, and Komiharu.
I'd say 12th and 15th are about equal -- 15th has a larger number of good members than 12th gen but also more forgettable members. Only one 12th gen was forgettable and she graduated super early, while 15th gen still has 2 and one graduated.
And the drafts for good measure:
1st Gen Draft: Hinana, Moekyun, Sayaya
2nd Gen Draft: Yuitan, Erii, Satone, Nao, maybe Kira, it's still too early to tell.