This is probably a very known one but Союзмультфильм is historically the russian animation studio. Nowdays it's not very good imo but their historic legacy is huge with a lot of exprimental animation techniques used and some really really beautiful classics. I've seen several japanese animators quote some of the Soyuzmultfilm works as an inspiration, so yeah, they used to be very very good.
Razpaz
This is probably a very known one but Союзмультфильм is historically the russian animation studio. Nowdays it's not very good imo but their historic legacy is huge with a lot of exprimental animation techniques used and some really really beautiful classics. I've seen several japanese animators quote some of the Soyuzmultfilm works as an inspiration, so yeah, they used to be very very good.