Well doing this by memory, may be a little wrong but those are definitely my top 10 most played (not necesserily what i consider best though):
1) Skyrim. Insane amount of hours on this one, keep playing with mods to this day
2) Heroes of the Storm I must have played around 8.000 PVP matches on this one over the years.
3) Fallout 4 Same as Skyrim, love it to death, i don't see myself stopping playing this for the foreseeable future
4) Witcher 3 I have completed this 3 times already, 2 of those were 100% runs with both DLC. I suppose this means around 300+ hours for all the runs
5) Empire Total war Steam says i have like 560 hours on it, but i am sure many of those were me having it open in the background etc. Still i played it a lot, definitely more than any other Total War game, and it is the only one i enjoy playing to this day. Loving the time period and early firearm combat.
6) Diablo 3 I have completed 3 seasons, completed the whole campaign (+ expansion) once, plus one half baked season that i didn't finish. I supposed this takes it to a decent amount of playtime, though i won't be playing it anymore
7) Fallout New Vegas . This is a strange game for me. I have completed it 3 times and have made several other characters and played many hours on them, but for some odd reason i never played any DLC, except the science fiction one, which i didn't even complete. Still i suppose in terms of hours i spend decent amount here
8) Fallout 3. Not as many hours as in NV, although i did enjoy F3 more than NV and i consider it a better game. Have completed the first 3 DLC to come out as well, never played Mothership Zeta and the previous one i don't remember how it was called. I suppose i played NV more because the campaign had more options to choose and thus i liked exploring those more, while FO3 was pretty straightforward, even if i enjoyed the locations/map more.
9) Civilization V . I have played this one a lot over the years. While i have played ever civ game since II, V was the first one that really clicked with me, mainly due to the combat being more wargame-like. Thus i spent more time playing it.
10) Xcom Apocalypse. This is the only oldie on this list, because i must have completed it many many times when i was younger.
Now that i am looking at my list, i realise something that tells everything you need to know about the gaming industry: Most games are relatively recent, released in the last decade or close to this decade. This is not just because those games are better, but mainly because i definitely changed my gaming habits as i grew older. When i was younger i kept playing like 10-20 different games per month, but most of those i only played up to a point and i only ever completed a few. But lately i stopped doing that and instead focused on re-playing games i enjoyed a lot rather than playing everything in the market for a little while. There are not many games that interest me anyway, i found out that 2012 was really the last relatively good year for me. Since 2013 i only ever found a few games per year worthwhile, and i think the last decent year was 2016. After 2017, it is a 4 year old drought for me. And it doesn't show any sign of stopping. I may be getting old and losing interest on games though. I am mostly interested on Bethesda games, plus a few 4X games, twin point hospital, etc, games like that. And i don't have much time to play those anyway, being a full time professional has its bad side, lol.
PS: If Football/Championship Manager was a single title, it would probably top my chart, since i have played it for many years since it was called Championship Manager 97/98. I am not playing it anymore as it has become too heavy and complicated and too time consuming and boring, but seeing as i have played it for at least 10 different yearly version, perhaps more, and i played it for at least 40-50 hours each year, some years much more, could maybe even top Skyrim for me.
