Let's discuss the games that we've sunk the most hours into.
My number one is Smash 4 without question. I believe I have ~1700 hours logged on my system right now, and I also have several hundred more hours from playing with friends and tournaments and streaming. So I'd say I'm close to 2500 hours total at this point. That's too many hours.
The other Smash games are probably a very close second, inorder of play time: Melee, Brawl, 64. I played a lot of Melee growing up, and after Brawl came out, I went back to Melee after a year or so. I have close to 1000 hours on Melee, around 600 on Brawl, and probably just over 200 on 64.
Pokemon is definitely the runner up series. I have about 750 hours on Pearl version, that's my most played one I think. Gen IV was the Pokemon renaissance for my group of friends. We all got back into it hardcore and played together, traded, battled and filled out our PokeDex. I also have a copy of Diamond with a couple hundred hours on it too.
Crystal version is probably the second most, then Red, then X, and all the others are just kinda whatever because I only played throught them once.
The Legend of Zelda is collectively probably the series with the most time logged. I've played through every Zelda game, even the NES ones. I know Twilight Princess is my most played because it's my favourite, and between the original and HD versions, I've completed it probably 6 times.
Second would be Wind Waker because doing everything in that game is hecka time consuming, especially on the Gamecube version without the swift sail. Then Ocarina of Time, I've done three or four 100% runs of OoT at this point. After that is A Link to the Past because I've completed it several times as well, and the rest of the series is just probably two or three playthroughs each.
Individual games I know I've spent a lot of time on:
Uncharted 3/4- because of the multiplayer.
Street Fighter V- This game is hard.
Mario Kart 8- I completed all the time trials and unlocked every vehicle part.
Saints Row IV- I've Platinum'd this game twice, once on PS3 and again on the PS4 Re-Elected version.
Super Metroid- So many playthroughs.
Metroid Prime 3- I 100% completed this game, and did a few speed playthroughs too.
Final Fantasy XII- This game is long as heck, and I know I've beaten it twice at least.
Mass Effect 2- Probably the game with the single most completions for me. I've done every possible route, made every possible decision, romanced every possible character. I'd say at least a dozen playthroughs on this one; it's a top 5 most played, definitely.