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There is no "best banner", it depends on the servants you have and servants you want

Have a look at the past JP banner lists and decide what servants you want and plan accordingly.

'Best' banners are those with single rate ups of the servant you want as it's the best chance at that servant. Doesn't mean you'll get it though.

Depends on what you want, this question is too vague to get a reasonable answer. I'll post some suggestions for starters:
Fate/extra CCC prep banner - Gilgamesh, Tamamo
Fate/extra CCC banner 1 - Meltlilith/Passionlip
Summer 1 - Caster Nero
Summer 2 - Lancer Raikou
Shimosa - Yagyu, Berserker Raikou