Good thing WoW doesn't count, because the hours would be silly.....
Off the top of my head, a couple games come to mind: FF VI and Star Ocean: Till the end of time. At least 150 hours on each, prolly closer to 200 for Star Ocean since I wanted to craft all their weapons and that damn 100 story building would piss me of by breaking my bonus chains. FF VI, I will always remember your little patch of forest full of instant-kill brontosaurus and meteor flinging t-rex. All in the name of Economizer and leveling up.
Legend of Dragoon was a long one too, easily over 100 hours spent on that. Ditto for secret of Evermore. FFXII surprised me by going over 100 as well, damn those stupid hunts and none-cooperating weather. As you can see, rpg's are my jam.
But for sheer overall hours in one campaign? Terraria....according to Steam, over 800 hours...oh wow, did I really spend that much time mining? Yes, yes I did. That's what happens when your girl looks at your square block of a house made of stone and stone wall and laughs at it. Now I need to mine all those sky islands and ake all those furniture..and wait what, there are new ones after that patch? In the dungeon you say? and new ore you say? Wait, and new PINK ore you say? Palladium ore was prolly the worst thing to happen to me....that and those blue furniture from the sky island, they're so pretty with the yellow accents and glass windows and banners....and those doors are so cute and inviting....