Many of the games mentioned are good, but have been surpassed (bit they may still be really good games to play) . Some classics here have yet to be surpassed.

One strong contender is Thief: The Dark Project.

Some other classics that have no rival:
Empire Earth
Stronghold
Witcher series
Baldur's Gate, Icewind Dale, and Neverwinter Nights series
Heroes of Might and Magic 2 or 3
IL-2
Vampire the masquerade
Freespace series
X-wing series
Morrowind
Beyond Good and Evil

Don't forget WARLORDS III DARKLORDS RISING. Unlimited replay value. Right on top along HOMM 2 & 3

I guess "more interesting" might be true, but I like Armed & Dangerous far more than Giants. It's funnier, and while simpler it's just more fun to play. Giants is a very uneven mixed bag of ideas.

I'd add the following to what others already mentioned

Bard's Tale (because it's funny as hell and just a joy to play despite it's simplicity)

SWAT 4 and TRON 2.0 because they are the two greatest FPS games ever made

The Heroes of Might and Magic and Disciples series, the two greatest strategy series ever

And Jedi Knight 2 and Republic Commando if you're a Star Wars fan.

Action:
Shinobi 3
Mega Man & MMX series
Contra: Hard Corps & Contra III
Saturn Bomberman
Rocket Knight Adventures
Bangai-O (DC)
Jazz Jackrabbit 2
Lode Runner Returns
Alien vs. Predator (ARC) & Dungeons & Dragons: Shadow over Mystara (ARC)
Star Fox 64
The Typing of the Dead
Super Fantasy Zone
Solomon's Key (Action Puzzle)
Front Mission: Gun Hazard (Action Adventure)
Wonder Boy III: The Dragon's Trap (Action Adventure)
The Guardian Legend (Action Adventure)

FPS:
System Shock 2
Strife

TPS:
Dark Forces 2 (you can switch between FPS and TPS in this one)
Bulk Slash
Sin & Punishment
Wild Guns

Adventure:
Day of the Tentacle
Loom
King's Quest III (the fan remake)
Snatcher

Horror, survival, zombie survival:
It Came From The Desert (Amiga)
Silent HIll 2

Strategy:
Zeus or Pharaoh
SimCity 2000
Starcraft
UFO: Enemy Unknown/X-Com
Dungeon Keeper
Heroes of Might and Magic III
Langrisser 2
Herzog Zwei
Rainbow Six: Rogue Spear (Stealth)
Shining Force 3

RPG:
Ys I & II Chronicles+ (ARPG)
Ultima VII
Terranigma (ARPG)
Baldur's Gate
Dark Sun: Shattered Lands
Fallout 2
Lunar: Eternal Blue

true ill give those a lookin too i am a semi star wars fan so yeah

I recommend:

* FTL: Advanced Edition on GOG.com
* Crysis® on GOG.com
* Homeworld® Remastered Collection on GOG.com

The graphics on all of those have or will age incredibly well. So well, I think I can call them timeless. There are not many games that will be just as fun to play 100 years from now as they were when they were first released.

https://www.gog.com/game/the_nations_gold_edition

Very good classic game. While the first game of the series is a poor Settlers clone, The Natios offer a lot of interesting mechanics and ideas which are not present in other games. You can read more about it here:
https://www.gog.com/forum/general/hidden_gems_recommendation/post1

Planescape: Torment.
Quake
System Shock.
Homeworld
Medal of Honor
Deus Ex
Command and Conquer
Prince of Persia series
Legacy of Kain: Soul Reaver
Age of Empires series
Rise of Nations
Starcraft
Hitman Blood Money

I think no one is going to suggest this one, but play Hogs of War :D

And as others said, I can suggest BG 1 and 2, Icewind Dale, Neverwinter Nights. Castlevania series tho they aren't on GOG.

Beside that I highly suggest playing Crash Bandicoot series on PS :D

I almost consider it as a non-game as there isn't really any skill involved, even if it pretends so. It pretends the game is about timing and such, but in the end your BALLS go quite randomnly all over the play area, and it is 99% a matter of luck and repetition, retrying enough many times that you finally are lucky enough and reach the next level.

It is as much a game as e.g. slot machine (one-armed bandit). If you consider them as valid games, then I guess Peggle is a game as well. A game of luck, sort of.

I play A LOT of Peggle but I need to agree with timppu on this one. It's barely, if at all, a game.

I also watch a lot of pro wrestling, but I'm not about to call that a sport. :P

I'd recommend:

Adventure games (pont & click):
-Monkey Island 1 & 2
-Indiana Jones and the Fate of Atlantis

Strategy games:
-Colonization
-Alpha Centauri

Since you like rpgs, Star Wars Knight of the Old Republic 1 and 2 are a must

For classic adventure I recommend Grim Fandango Remastered and Day of the tentacle remastered

For strategy you could try the older master or orion games

Die by the Sword with some mods.

https://www.gog.com/forum/die_by_the_sword_gold_edition/die_by_the_sword_mods/page1