I remember I wanted to try Arcane Dimensions, too, but something stopped me from doing it. Not sure if I didn't get it to work or whether I was put off before even trying. Or maybe I discovered it only after Quake Enhanced and learnt it's not quite compatible to that one, and I didn't feel like tinkering with sourceports again?

I don't remember which sourceports I tried though, could be it wasn't vkQuake but something else, and part of what made this exhausting for me was the agony of choice, not knowing which one was the best. But yes, the major complications were probably caused by having to patch the music back in.

At least with vkQuake, you just put the Arcane Dimensions folder in the Quake folder and select it from the Mods menu, not anything else to do.

Anyway, that's rather off-topic. More games 15+ years old when I played them: The first few Age of Wonders (a little clunky in spots, still very playable), the first couple Anno games (mostly good, though I didn't like 1503 as much as 1602 and haven't played any more), Earth 2150 (played the Amiga version of 2140, was OK but very generic, 2150 is definitely better but there sure is a lot of it, haven't finished).

Might and Magic 6, 7, 8
Heroes of Might and Magic 2, 3
Star Wars Knights of the Old Republic

Warlords Battlecry 3

Fatal Frame 1
Fatal Frame 2
Fatal Frame 3

Fatal Frame: Maiden of Black Water Inspired me to play those games. Since i still have working ps2, I acquired those games from Ebay. Second Fatal frame is my favourite one!

Clive Barker's Undying

Joe & Mac caveman ninja

Blade Runner

X-com terror from the deep

Gabriel Knight 1

Gabriel Knight 2 The beast Within

Gabriel Knight 3: Blood of the Sacred, Blood of the Damned

Realms of the Haunting!

Daughter of serpents

Harvester

Lure of the Temptress

Flight of the Amazon Queen

MechWarrior 4: Vengeance

MechWarrior 4: Mercenaries

I meant the one with ice monsters instead of floating eyes, and similar variations.

Back to the topic, I've also played for the first time (after they were released on GOG) and enjoyed: Sword of the Samurai, Full Throttle, and Sam & Max Hit the Road.

Well ... most of my favorite games are this old now.

Like, my favorite game of all times, Vanguard: Saga of Heroes, was released 2007, so thats 17 years ago. It shut down in 2014 though, so ever since one cant play it anymore.

Other such games are Baldurs Gate 2: Shadows of Amn (2000), Star Wars: Knights of the Old Republic (2003), or Vampire: The Masquerade: Bloodlines (2004).

But the OP maybe means games that we played 15 years after release FOR THE FIRST TIME ?

The game that I played the longest after its release was No Man's Sky, I think I bought it 2021 or 2022 when it was released in 2016.

So I cant say I know any such game.

For me it was Curse of Monkey Island. I skipped in the 90s because stupid younger me didn't like it was different than the previous two (mostly its graphics). Luckily when I got back into classic games with GOG I corrected that mistake.

Just off the top of my head, here are some pre-2003 games that I liked, but didn't get around to playing until long after their original release (or if I did, it was only for a few minutes, maybe a few games or the first levels, but nothing serious until much later):

Gothic
Nox
Arcanum
MDK
Giants: Citizen Kabuto
Silver
Desperados: Wanted Dead or Alive
Robin Hood: The Legend of Sherwood
Blake Stone: Aliens of Gold
Doom 64
Unreal
Serious Sam 1 and 2
Phantasmagoria
Toonstruck
Tex Murphy Mean Streets
Flight of the Amazon Queen
Sanitarium

Arcanum: Of Steamworks and Magick Obscura (game from 2001) with UAP (Unofficial Arcanum Patch) is playable and quite enjoyable :)