My character that got most close to broken was a Master Of Many Forms druid, though I was playing with a group with two well skilled min-maxers who were ridiculous from the outset at level 3

Wish can’t make you great, it can’t do much more than the equivalent of about half a level, you need a broken character design from the start

Of course there’s also support for epic level progression taking you beyond level 20. A druid at level 20 could face an army and win