kore wa imamade de saiko ni tanoshikatta desu. これは今までで最高に楽しかったです！
”最高 saiko" means highest degree, peak, etc.
FYI, there is a slung as well that I never say but Japanese teenagers who are fashionable and like hip hop style say " バイブスあがった！(vibes agatta!) Vibes is a slung in English as well, right? "Agatta" means "went up", but again, I never say this..haha.