Defend Homeworld ends up in pretty much every deck, but for different reasons. If I'm using any Ref stuff at all, it's an auto-include since it's the one Ref card that's useful in every game. Sometimes it sets up a play or draw engine (e.g.
Telak or
Deyos). Sometimes I have a bunch of great SECURITY personnel and I want to be able to download the right one for different situations. Sometimes I'm using
DNA Security Scan.
The original
Q's Tent is another. I hate being forced to attempt a mission with 4 personnel when I can solve it with 2. If I think I need any Ref cards I can still put them in my tent or even in my draw deck. I also often stock Tents in my draw deck, and IMHO it's the most underrated card in the game (though it certainly doesn't belong in every deck). I'd rather play a game where I can access powerful situational cards when needed than play a game that's just about spewing out the highest volume of random cards.
I'm a Doctor, not a Bricklayer shows up a lot because it's a personnel download I can use during my opponent's turn.
I use
Cytherians a lot because it's fun, effective, and I don't have time to come up with any new dilemma ideas.