Tough choice but ruling out free games (if we include emulation I'll be here all day) I might go with Invisible Inc. It's a turn based stealth game with lots of different characters to try, each leaning into different strategies. While it's possible to knock out or kill guards, it's discouraged because it increases the alarm, so you have watch patterns and use tricks and diversions to achieve different objectives. I have 200 hours on it and it's on sale rn for $5. There are other games I have more hours on, but not as much enjoyment.
Runner up: Heroes of Might and Magic IV. I have no idea how many hours I have on it because I bought it before I had Steam, on a compact disk (in fact it's not even on Steam, I think it's abandonware?) and have come back to it so many times over the years. It's kind of a timeless hidden gem, lots of ways to build your characters and your armies, lots of different challenges, and the writing is fun and memorable. The style is a little different from the rest of the series and I never got into the others but the new one (Olden Era) is promising (and has a free demo that's really good).
Worst for me are the Total War series, maybe Napoleon or Rome 2. I remember when the first ones came out and there wasn't anything like it. In the year 2000, the ability to fly around the camera ordering troops around was incredible, and of course the ninja assassination clips were absolutely peak. But since then, every one pretty much follows the same formula, and it feels like if you've played one you've played them all. It's not that they're bad games, I just couldn't get into them.
