Depends to some degree on whether I'm playing by myself or with my multiplayer group and what the game settings are.
In multiplayer I'm "scout guy", so I always end up with mk3 of those and am in charge of prioritizing our future targets. I still usually get them in single player, except they'd be a bit silly on something like a 10-planet map.
In single player, the economic command stations are hard to say no to first thing, because the resources are just too helpful, and there's usually somewhere safe-ish to build them. In our large multiplayer games, a lot of the time we don't end up taking a ton of planets total, and we don't end up with very many each, and they often are somewhere that needs the better protection of a military one, so that's a bit more questionable.
The mk2 ships almost always get unlocked early. Bombers are mandatory, and so are fighters, depending on your bonus ship. Force fields happen eventually as needed. Flagships are pretty common early. Also a fan of gravity turrets. mk2 engineers are not uncommon at some point in the game.
I used to always get spider turrets really early in the game, but I kept killing my economy with them, so I've found other ways to deal with things and don't unlock them at all a lot of the time now. I also used to use tons of Raid Starships back in the day when they were the scourge of the galaxy, but I don't think I've spent much knowledge on them in months at this point.