If I have them unlocked, low-ppowered planes. yank em into full power right by the cmd station. If not, I send as many units as I can spare and hit posts, starting with the spec forces post.
LP-cloaked Space Planes ++. They're fairly inexpensive and will rip specific targets apart in sufficient numbers (100 or so Mark I's will instantly destroy a Command Center). Just make sure you get all the lightning turrets and AOE ships at the wormhole where they'll be entering the system (or the tachyon beam generators, but a fair number of high-mark enemy planets will have cloaked ones as well), as otherwise they may all die instantly. Also don't attack SF guard posts or targets by fortresses unless you've got many, many planes, because you will lose them very very quickly.
If the planet has an Ion Cannon and Mark III tractors, and you lack a cloaked or teleporting ship that can deal good damage to structures, you're best served using transports. They don't cost much knowledge and can save you ridiculous numbers of losses, and there's no cap so you could theoretically send in 20-50 decoy transports into a very strong enemy system along with the ones actually carrying troops.
Early-game when you're attacking low-level planets, sending 10-30 fighters through to eat the tractor beams and then bomber-rushing the Command Center works just fine. I usually move the bombers to the outskirts of the system afterwards rather than trying to bring them back into the wormhole, because that reduces losses quite a bit.
I generally never include frigates or starships in these attacks because they're too slow, although it could work an enemy planet with no long-range dangers to speak of and early on Frigates a necessity for taking out SF guard posts. It would just take extra time, which you may or may not have. Note that this is a moot point if you use transports, since anything that can board a transport suddenly becomes ridiculously fast. Also note that Raid Starships are in a class of their own, since many players use them to rip through multiple enemy systems with no support whatsoever.