Always free games you have the most playtime

TF2 1700+
CSGO 1350+

Dota 2 with over 7k hours, haven't played it for 3+ years now though. The next is probably CS 1.6 with about that much across all my previous steam accounts. :D

Dota 2 , nearly 17k hours

Rocket League 750+ hours
If I had played Star Wars Galaxies via Steam, it would be that. Edit: Nevermind, was probably not permanently free even in the end.

Have to think if I played any F2P before Steam for a long time. But since I can't remember, I don't think i did.

According to my Steam Library I played:

Puzzle Pirates for 1300 hours
No more Room in Hell for 200 hours

Dota 2 with 2121 hours... :|
Outside of Steam, I remember my RuneScape account said I had played for 123.4 days (and, yes, it was exactly that number- that's why I remember it)... so over 2960 hours...

I'm not sure which one is worse... Fuck both of those games. :D

The 4 free/F2P games in which I've spent over 100h and I do not regret doing so:

RPG MO - over 2k hours of active play (and many more of idling in the background). Do you like original RuneScape alike grind and graphics? Just two devs from Estonia run this game with constant updates for over 8 years. No content locked beyond a paywall. Pay for convenience in-game purchases, all of which can be repurchased from other players on market. Very small (a few hundred people active on chats), civilized and helpful community, with excellent player-moderators, which also always are ready to give advice to newcomers, if it doesn't come from other players (and, yes, probably close to 50% of negative Steam reviews comes from the fact, that the community does not tolerate rude comments and trolls, who persist in posting such comments).

Soda Dungeon - 293 hours. Very simple, yet surprisingly enjoyable. I did 100% it with no regrets.

The Pirate: Caribbean Hunt - 262 hours. The most enjoyable real-time naval combat system, that I've found until I began to play Rebel Galaxy.

Pixel Puzzles Ultimate Jigsaw - 193 hours. If you enjoy Jigsaws, it's a must. Excellent engine implementing even little known jigsaw features like the uneven cut. Lots of free puzzles, large monthly free "challenge" puzzle, bonus points to get some more stuff and if you run out of it also pretty cheap/easy to win or buy in bundles DLCs.

Honorable mention goes to Path of Exile, which I've played out of Steam for quite a while. Possibly still the best ARPG around, if you are OK with economy based on player to player deals with no in-game market and either severe inventory restrictions or necessity of purchasing additional storage space. Those however are long term concerns and the game should be very enjoyable without addressing them for the first 100 hours or so.

Blush blush and crush crush - no idea how much bc I managed to delete them hides

Pixel Puzzle Ultimate - 326.1h
Is it really free tho? Have a few packages, so debatable thinkin

Brawhalla - 1238h
Idle Slayer - 1057h
MINDNIGHT - 799h
Town of Salem 2 - 587h
Trackmania Nations Forever - 340h

War Thunder - 1200+h
Crusader Kings 2 - 473.5h

Enderal: Forgotten Stories (Special Edition) 250 hours + ~100 in the original release Enderal: Shards Of Order (2016)
Planet Explorers 192 hours

Free games that I play are mostly standalone, so I don't keep track of time spent.
My top favorites are Angband, League of Legends, Postknight 2, Slice & Dice, and The Battle for Wesnoth.

Genshin Impact - Several thousand hours at least (on PC and Android)
League of Legends - Few thousand hours at the very least (It's quite fun but just really time consuming so I'd rather play something else now)
Rocket League - 1600hrs (Uninstalled long time ago and never coming back lol)
Honkai: Star Rail - 1000+hrs (on Android)

Team Fortress.

BONK!

War Thunder 2,246.4
Warframe 1,505.6
*Black Desert 1,143.7
PSO2 NG 483.9
Lost Ark 461.9
Destiny 2 326.2

*Yes I know Black Desert is not free, but it has been very frequently given away for free.

I have tons of idle time in all these, I think most people do as well in online games.

How do people get thousands of hours in a game? Over a decade I probably don't even have such an amount of hours in all of my Steam library as some people have in one game.... Do you game EVERY evening from 5 to middle of night? EVERY weekend? Or don't you work? Like seriously it baffles me.

EDIT: First free game that showed up for me was Fallout Shelter. Which I've played for about 50 hours. On Steam.
I also checked my phone games and there I have 100 or so hours both in Simpsons Tapped Out and Candy Crash.

7+ k hours of
https://store.steampowered.com/app/329110/Gems_of_War__Puzzle_RPG/

rookie numbers...
destiny 2

top 3
1 - crush crush - 4.5k hours
2 - adventure capitalist - 3.7k hours
3 - blush blush - 1.8k hours

I always forget the way to check the free games played. They don't show up at the browser.
I must have played around 40 to 50 hours of Zombie Panic! Source.
Around 25 hours on Double Action: Boogaloo.

i got like 7k hours in dota 2 between my 2 accs

Phantasy Star Online 2, 1750hours.
Probably close to 2000 since I started on it back when the only way to play it was before it was on steam, with the japanese version which had a rough english fan translation on the japan servers, the lag and disconnections were real hard.

Next to that is Warframe at 1100 hours. Similarly, probably 2000hours since I played it outside steam before.

Around 1200 hours in Crusader Kings 2, although most of my playtime was when it wasn't "free" yet and I technically bought it with most of the DLCs so I dunno if it really counts :x

662h Alien Swarm (superseded by Alien Swarm: Reactive Drop)
472h Team Fortress 2 (originally a paid game)

Unturned, 2 Accounts - 500+ hours