Haven't gotten around to levelling my prestiges but I'm probably going to try the Raynor, Artanis and Karax ones first for funsies (Raynor ones look solid across the board and open a lot of playstyles for an already engaging, deep commander, Artanis' warp mastery, I am told, stacks with the benefit of Nexus Legate, and I hear funny things about Architect of War supposedly working with ESOs and other commanders' static defenses).

Most overrated so far is probably Abathur's infinite Ultimate Infestations simply because you can already win most missions with just the 6 UIs and slowing down the tempo on getting your first Brutalisk out can be brutal even with perfect biomass farming. I guess I am also disappointed with Swann masteries overall as I like playing Swann.