Skyrim, until my saves met its demise due to several glitches. It ate my companion, whom I was really attached to (Farkas). When I managed to recover him, another glitch ate him again. I made the mistake of clearing saves files somewhere in-between, so I lost like 500+ hours of gameplay. I was mad. Still am, I haven't played it much since.
Guild Wars ate about 15,000 hours of my life. I did everything, except PVP past Rank 3. Collected mini-pets. Powertraded like a madwoman. FOW armors and Tormented weapons across my entire characters, except my ranger. My necromancer alone - my main - was worth quite a nifty amount.
I don't know my hour count on Perfect World International, but across my 3-4 accounts, it swallowed years as well. Played on and off since launch, up to early 2015.
Wildstar is eating up a lot of my time, between raiding, leveling alts and RPing. I can't play it right now, so it's falling onto...
Pokémon. I have around 500+ hours on Y alone, where I MM, hunt shinies and fill my living dex. I spent about 200 hours on my original Gold cartridge, most of those dedicated at filling my Pokédex.
Mortal Kombat X killed a good amount of time, I have 200-something hours clocked on the PC version, and I don't know how much on my PS4 (granted I leave it on and check on daily/hourly living towers these days).