While the Rock and Cena are obviously ridiculously strong, don't sleep on Seth.
There are quite a few different types of medals, which can make certain characters more difficult to raise.
KOF souls can only be used on KOF characters. All collaboration or other series characters use the All Stars medals. Bronze medals can only be used on bronze category characters, etc. The easiest medals to acquire are KOF gold souls. The other souls can only be found in certain events and festivals. The Soul Festivals are the best ones so don’t miss it! When Roulettes appear, unless a certain type is needed, the best strategy is to go for the bingo for the most rare drop.