Steam

Warframe is a good one. After 4000 hours I got burnt out of some of the things it does poorly and haven't played in a year, but overall it's fun with crazy new content on a regular basis for free.

Terraria has far more depth than I ever expected, and way more than that added since I last played. This game can sustain a player for ages.

I would say Satisfactory but I did actually get tired of it for a while. Itching to go back with the 1.0 release though. I love it but the two above have better longevity.

Beam.NG would be one just because it covers nearly every driving discipline and the other would probably be Cyberpunk. It’s not even that Cyberpunk is my favorite, but it has so much content in it at this point that it would keep me busy for years.

Torchlight2 and totalAnnihilation

Rimworld and M&B Bannerlords

I really wanted to fit Crusader Kings 3 in there, but I think ultimately these two cover all the bases, and have an amazing modding community to keep it fresh and interesting.

I only need one and is called "Space War" 🧐

Dwarf Fortress and Dota 2

Monster hunter world and satisfactory

Quake Champions and Monster Hunter Rise

G-Mod and ARMA 3

Oxygen Not Included and Baldur's Gate 3 for the 1 month out of the year I take a break from ONI.

No Man's Sky and Light No Fire

Path of Exile 1 and 2. Not much would change

Slay The Spire for replay value and Rocket League for "pvp" itch.

Terraria and Starbound bc mods

I have a very specific taste

Battletoads and Frog wars.

That is a hard choice

Arma 3 and GTA VI

Stellaris is number one and Skyrim or Fallout 4 for number to just based off mods available.

Skyrim and Witcher 3. Shoutouts to Baldurs Gate and Witcher 2, but the latter is far too short for me to want it on my "you can only play this" list, it's quirky but there's something special as hell about it for me. I've played it through about 15 times.

Vermintide 2 and Tekken 8.

Both games have high skill ceilings, so I could probably spend years (and have) playing both and still finding ways to improve or optimize gameplay.

CS2 & super auto pets

Dwarf Fortress and No Man's Sky. I wanted to say Satisfactory instead, but felt it might be too close to Dwarf Fortress in nature and that would make it hard to want to play either game if I was burned out from one of them.

Path of Exile for sure, not sure about the second though. Maybe Dark Souls 2, Elden Ring, or maybe XCom 2.

Path of Exile and Rocket League

The store tab and the library tab

Satisfactory and Titan Quest

Rain World!!!!!
Second place is hard to do though! Oxygen Not Included, I guess?

Nioh 2 and Satisfactory

TF2 and TF|2

Skyrim and Half-Life 2

So I'm going for longevity. These two definitely wouldn't be my first choices for top games.

Elder Scrolls Online and Minecraft. There is enough story, social, and creativity that I would hope it works out.

Once Baldurs Gate 3 gets solid modding, and assuming modding is allowed, that takes a spot.

Satisfactory & Mass Effect

Assetto Corsa (assuming mods are included) and Factorio (again, with mods, but to a lesser degree).

Apex and Rimworld