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.  

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.

Wizardry I

MOM

MOO2

Going back away Beyond Apple Manor

HoMM2,3 & 4

AD&D Gold Box series

Bard's Tale 1-3

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

MOM

MOO II

AOW:SM

Warlords II

Civ II

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.

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).

Only one. Choose.

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.

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

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

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