SE4 is the one I am playing. It has rich strategic depth and is everything an empire building game could be and more. It's so good it's addictive and worth the $20 I got it for.
It has 2 parts. The empire building and then tactical combat when two or more enemy empires ships/fleets get sent after each other. They are both turn based.
In the new SE5 due out this year, I thought malfador, he's the coder, was gonna do the tactical combat like the OP engine. Would be spectacular.
I've modified some of the SE4 files so that it's similar to trek. Warp engines, phasers, drones, Romulans have cloak, no one else does but can research it, etc. Cool stuff.
Cant wait for SE5. Be nice to see what the 3D tactical combat can do. I was thinking maybe they'd use real low polys and the OP engine could handle a few hundred ships. But maybe they are gonna use some other engine. They could do so much more with the OP engine tho...