Well, Sins of a Solar Empire and GalCiv II: Dark Avatar are my current favorites. Yes, I still play GC2 and will continue to do so, it's amazing and still leaves me amazed at what can happen in that game.

Older favs would be (not all being quite 4x but close)
Master of Orion 2
Stronghold: Crusader and Stronghold 2.
Homeworld
Couple games based on Roman times like Civ/Simcity, don't recall the names

Galactic Civilizations II, and Heroes of Might and Magic II and V. I haven't played any of the other HOMM games, but in all seriousness, give them a shot if you like fantasy 4x. Really fun combat.

Imperium Galactica (the first one) was my fave. I still to this day haven't had an epic ridiculously long impossible campaign in any game since then. Shame it doesn't run properly on XP.

I'm also one of those strange people who liked playing Pax

I didn't like pax imperia 2.

I never got to try either but wanted to. Both looked very promising, and even clunky UI's I can look past if the depth of gameplay is there.

Gal Civ 2, Startrek Brith of the Federation, Lost Empires Immortals.

IG2 was a lot easier and shorter, but looked much nicer. They were both a mix between sim city (building up your planets in sim city style view), a slow C&C (where you'd do battle on the surface in between the cities with your fleets ground forces and planetary defences) and the space battles were a bit like GalCiv. Research tree, no control over your forces, but you could customize your ships. The starmap was realtime too, but slow enough to give you anything up to 40 minutes early warning of an impending attack on a large map.

Ahh the memories. Took me three months to complete the largest possible IG1 game. IG2's biggest could be done in mere days. Still worth picking up either one though off ebay maybe. Even if only for the spacey music tracks.

IG1 on planet screeny Not bad for its time.

My all time favorite 4x game, in fact it's in my top 10 games of all time, is Alpha Centauri. Though I have to admit Sins may knock it out of my 10. Or at least it will knock something out of my 10. As much as I have been playing it is still to early to say. Top 10 games stand the test of time and Sins is still young.  

GalCiv 2 and Star Trek Birth of the Federation. Both games see quite a bit of playing time on my computer.

Alpha Cenaturi, Sins and Galciv II

Sadly never got MoO 2.

Didnt like MoO 3 cause it did not feel like a game.
Didnt like IG 2 cause there was no AI on "easy" and the Ai on "Medium" was already a cheating champion.

MoO1 is nice even when played today on a dos emulator.

Ascendancy was an amazing game...
http://www.abandonia.com/en/games/221

It's very close to being a turn-based Sins..

Wow, I remember that game. I played the heck out of it. I hadn't thought about it in years. Good call.  

The best 4x space game is undoubtedly Space Empires V... No question.

Though the game is much, much better vs. multiplayer than the AI.

Javaslinger

This is still my favorite 4X. I really liked the terraforming options, You could raise and lower hills/moantains. Create river`s, Planet buster`s(Nukes)actually blew massive crater`s into the ground.

SE:IV *still* gets regular play from me and some of my friends (I just showed it off this morning to more potentials along with Sins).

I like SE:V but I feel like I can't get an overall picture as quickly. It's probably more of a case of just spending more time with it. I do like research in SE:V, it's more "realistic" per se than the way SE:IV does it.