Old xcom(tftd) and ufo

Final Fantasy Tactics

Tactics Ogre

Gladius

Panzer General series/ Fantasy General

Age of Wonders

Master of Magic

Silent Storm+ Sentinels+ Hammer & Sickle (why they dont make more games on this engine...)

Gladius probably had the best turn based/tactical RPG combat of all time.  The game didn't have much more than that, but it's tactical combat was incredible.  And your roster of 20 could be formed from all types of different warriors, I loved that game.  

Master of Magic, tightly followed by UFO:Enemy Unknown (which you probably know as X-COM) and then Alpha Centauri

Honorable mentions to Civilization 2, Warlords (the original), and HoMM 3.

Wow, hard to pick just one! Some excellent titles already posted (and I played most of them way back when).

All of the KOEI titles were great fun as I recall (Nobunaga's Ambition, Genghis Khan, Romance of the Three Kingdoms...)

Well, narrowed down to 2...

X-Com: UFO Defense aka UFO: Enemy Unknown

Master of Orion II: Battle at Antares (MOO2)

I'd have to say X-Com only because I enjoyed the squad based game play a little more than MOO2's space combat and there seemed to me to be a little bit more tension created during the night missions with help from the creepy soundtrack.

Masters of Magic hands down! Also my favorite game of all time (playing computer games since 1987)

Steel Panthers as the turn based tactical leader

Honorable mention:

     Warlords 2 and Civilization 3

Being able to have mega-armies with hundreds of units is my top appeal for play, as in these games. As the graphics improved the ability to deploy larger numbers of units seemed to have waned. Perhaps as more 64bit PC Strategy Games are attempted it will revive the genre. The formula for GC1 is still more appealing to me than GC2 for the same reason but I still like and play both games as they are excellent.

Elemental, both WoM and Fallen Enchantress I do enjoy playing at this time and I believe that WoM is an excellent attempt at recreating MOM and I will enjoy its replayability for many years to come.

Wizardry I

MOM

MOO2

Going back away Beyond Apple Manor

HoMM2,3 & 4

AD&D Gold Box series

Bard's Tale 1-3

for its time the original Gemfire for the snes was not too bad of a game.

 Lots of great games mentioned here, but I'll say HoMM 3.

Civ 2 

Way above everything else

MOM

MOO II

AOW:SM

Warlords II

Civ II

Sword of Aragon, CIV 2, HOMM2, Alpha Centauri in no particular order. There are probably other better games but these are the ones that I remeber playing an aweful lot when life was still simple. i.e., No wife, kids and business :) 

Oh, this is hard... the BEST turnbased strategy game... thats hard... but, I have to say:

Master of Magic

Honorable Mentions:

Romance of the three kingdoms 1-4 (pc), VI (ps1), VIII (ps2), XI (pc) (well, pretty much anything made by KOEI)

Heroes of Might and Magic 1-3 (with expansions +WoG (wake of Gods))

Master of Orion 2

Sword of Aragon

Warlords 1 and 3 (+expansions)

Centurion, Defender of Rome

DragonsBreath (aka. Dragonlord)

Supremacy

Oil Imperium

Birthright Gorgons Alliance (wish they would release a new version/sequel)

Millenium 2.2 (aka Millenium Return to Earth)

Deuteros

RoadWar 2000 and RoadWar Europe

Well, those are the games I STILL play even today and enjoy, alot. There are probably more that I havent mentioned, but those are the ones that came into my mind when I read this thread. Remember, a good game isnt all about graphics and stuff. Its all about the gameplay and the joy you can get from it.

For all of the MOO II lovers, check out StarDrive (the beta is on Steam). Lots of MOO II influence and a great game in the making. Hyperspace Flux, Planet anomalies, Bultrathi (now called Kulrathi or something). I have played the beta a little yesterday and it is very promising. There are lets play vids on youtube (from the master Das himself). 

I'm going to say GalCiv2. Though my combat/multiplayer favorite is Sots 2 (yes hate me but no one does epic pew pew like Sots 2)

Honorable mentions Civ 4 & Civ 5(with civup & gem), Sots 1 & 2, Alpha Centauri (need more games like this one hopefully Pandora lives up to the name and I just recently played AC old but golden), Rome Total War (hopefully RTW II), FE (its growing on me so keep those expansions coming), and Star Wars Rebellion (yes lots of rough edges but it still had some good qualities). 

Actually now that I think of it Rebellion AI does something the other games AIs don't  It ramps up. You select easy, it easy at first but near mid game the ai slow makes its way to hard ai and then near the end its on hard (though its too late).

Imperialism 2

Only one. Choose.

Am I forgetting something? 

I'm pretty sure Warlords was better than the sequel Warlords II.  Focus was on units rather than heroes.  Warlords was better because you were pushing with large armies on numerous fronts.

Christ, David Bowie has a new album out, first in 10 years, and I was remembering the times then. I started feeling old. Now you post THIS. Are you trying to give me a heart attack?!
Empire was rad. Samw with warlords, for the time period they came out that is.

SMAC without question.

Stardrive seemed interesting until it said it was an RTS. I find RTS empire building games irritating. Alas.

Master of Magic. Honorable mentions:SMAC, Civ IV

Another great game (and really hard to beat the AI)

Please. Those of us who played Temple of Apshai on a TRS-80 know better.

MOO2 & CIV4BtS for me as well.  Two games that just kept bringing me back again and again over the years.

Wow...MoM sure tops many lists and Xcom is up there as well.  I know Stardock tried to get the rights to MoM and that fell thru, but you would think that someone would say "Hey, with the success of the newest XCOM, why not redo another classic that has quite the following?"  Im enjoying FE and where its going, but that thought keeps going thru my mind.  Then again, im not a programmer, or what not...just a gamer who likes to play fun games, especially when im out in the field for weeks at a time.