Marza or the [kol analogue] for the vasari. those are the clear forerunners of the two races. the carrier is nice for a specialized form of slog and kite tactics, but the marza and [kol analogue] are the only ships that have the power nescessary for you to move quickly through planets and to bring the power nescessary to head to head combat. the Kol becomes effective at later levels, however it cannot compare whatsoever to the Marza when it matters: early on. as for the Vasari, despite the lower cost of the [kol analogue] its actually one of their most powerful ships. early on it has the life and strength nescessary to move moderately quickly through enemies while surviving a head on confrontation with enemies. at later levels you can use it together with a concentrated attack to completely erradicate a section of clumped frigs, incredibally powerful.
other ships have niche uses, however currently the Marza and [kol analogue] are the most powerful well rounded ships. carriers do *not* bring the power nescessary to combat and are almost explicitely used in the backup of other ships or to rob your enemy of income. the Kol is useful for extended battles where the longer you're alive the better (say, for defense) and the colonizers are good for early expansion, but generally aren't worth their counterparts.
Does anyone attack a planet without having a capitol ship in there fleet?
depends on the situation, if my enemy is bunkering down and forgetting to defend certain key planets then I will bypass him entirely. capital ships are more or less for the kicking power nescessary for extended battles.