I enjoy X3, like JD, bought Terran Conflict and have yet to really get into it (backlog you know). The mouse control is great (I prefer it this way as well), although it takes a bit of a computer to run it super well. You can do nearly anything in this game, build a trade empire, buy/own/operate space stations, go to war, buy large warships, own an armada, etc. It is a lifetime game, like WitP, as you don't ever finish it.
I really enjoy Starships Unlimited. It is old school and a tad simplistic, but has lots of charm and works rather fast. It will suck you in as you play.
SotS is the one of the finest, IMHO, of 4X space games, particularly if you want lots of combat. One should get Gold if they have the choice, as the stock game is inferior to Stock + Born of Blood (BoB). I have not gotten to really put tons of time into A Murder of Crows (AMoC) but what I have done really looks wonderful. The game is much more complex now that its relatively shallow wargame focus in stock release. AMoC + BoB has all sorts of additions, diplomacy, trade, cultural domination, fighters/carriers, six races that play completely different from one another (each is very unique, has certain weaknesses and strengths, different propulsion, and different random tech trees). The random events are huge show stoppers in many cases.
I love the Gal Civ series as well. Much more like Civ and very challenging.
"Neca eos omnes. Deus suos agnoscet!"
(Kill them all. God will know his own.)
-- Arnaud-Armaury, the Albigensian Crusade