MoM and Alpha Centauri. 

The undisputed masterpieces of the genre. Flaws and all.

Choose, grasshopper, between them. For only one will guide you forward.

I think maybe the Battle Isle series kept me occupied the longest.

They are doing a remake here.

http://battle-worlds.com/

Pretty excited about this.

Civilization series, GalCiv2 and FE; Legendary Heroes! Don't care for other games actually, but Europa Universalis series of course (but it's not turn based, just slow paced rts grand strategy).

WOW!!!!

I can't believe it hasn't been named yet.  For anyone who has played it, ANYONE, Dominions 3, is hands down the best TBS yet made.  I do not say this lightly.  Comes from Shrapnel Games one of the masters of the genre ... and a non-trivial price.  I am really a dyed in the wool sci-fi MoO, GalCiv, etc. and the historic stuff.  So, for me, it came out of left field.  Simple premise, you are a wannabe god ... but there are other wannabe gods as well.  The goal be the only god left.

You have people that worship you, a race.  The race has unique attributes, advantages, disadvantages.  Maybe 60 races, 1500 units, 600 spells and 300 magic items.  The diversity of this game is unbelievable. There is no "way to win" especially in multi-player.  It's all situational.  It's a little rough on graphics, but on game play, it has no peer.  Keep in mind, I am NOT a fantasy player.  But this is in an entirely different league. 

If you are a student, dump it off your computer entirely. it will suck an entire semester out of you.  Steep learning curve, I am still not sure if I know half the in's and outs of this game.  But despite the curve, it is still very playable without a deeper understanding.  Although some stuff will happen to you that you have no idea how.  Anyway, addictive ... and fantastic.

The best unmodded game would be the Master of Orion 2 for me. But there are plenty of mods that make other games even better than vanilla MoO2:

Age of Wonders 2 (Shadow Magic) with Brave New World mod

Jagged Alliance 2 with any of the big modification based on the 1.13 unofficial patch mods (Urban Chaos, AIMNAS etc.)

Civilization IV with Realism Invictus mod

Dominions 3 with bunch of different mods

SMACX OF COURSE!!!!

I probably spent the most hours playing MoM, HoMM 2 and MoO2... but if I could only pick one TBS game to play forever and ever, it would be HoMM3.

Civ 4 BTS + BUG +BAT

From what I hear, if I ever tried rule changing mods, I would love FFH2.

3 way tie

Sword of Aragon- If this game got remade with some design polish and multiplayer, it would be fantastic.

Moo2- Can't mess with the classic, but needs better tech balance, because most of the time the decision is always production / research tech. Imagine MOO: Battle at Antares II- A tweaked remake of the original with the inclusion of Moo3 races (heh, now imagine Moo II with Antarans attack and all the while the Harvesters are festering in the dark corners of space waiting to pounce) 

Battletech: Crescent Hawk's inception... as a teenager I was waiting for all these fabulous turn based order phase / real time execution phase games to be coming after loving this game to death. Derp, cant believe there aren't a crap ton of triple AAA games in all genres using this system. This game also had it all but was shallow in content... imagine it with all the bells and whistles of a fully fledged battletech content implementation and multiplayer... and no, mechwarrior tactics is still a giant turd compared to this game.

maybe he has 3 systems running and plays 3 games consistently at the same time^^ left eye left monitor, right eye right, whats going on middle monitor you would lern to smell instinctively over time^^  

would need some extra arms though  

CIV IV

Civilization II....One of the best strategy game ever for me... I have played this game on my play station!!!