関の山（せきのやま） is the best you can expect.

「関の山」（せきのやま） is used for something that is the best, most, highest, biggest, etc. you can expect or do. That’s the utmost thing you can do.

The expression comes from a large vehicle 「山車」（だし）which is decorated for a festival and carried in a parade in the town of Seki 「関」（せき）in Mie Prefecture 「三重県」（みえけん）.

The vehicle was said to be the best at one time.