I would have to go with Golurk, since I found it to be more useful when playing competitively. It has access to Stealth Rock (one of the most help entry hazards), good defensive stats, a wide range of movepool, and an amazing Attack stat. Thanks to its Iron Fist ability, it's able to land some of the most devastating punching moves and knock out nearly any Pokémon with only one or two hits. d: Its Speed stat is a bit low, but if it was higher, then this Pokémon would have been almost impossible to stop.
I really Gigalith's design, but I don't use it at all in any of my battles. Because of its weakness to water and fighting (two of the most popular Pokémon types in competitive battling), you can't do too many things with it. You should be able to set up a Stealth Rock or two with this Pokémon, but it'll be taken down by your opponent before you even know it. D: