Obviously, start with Gibbs, he is much better than the loser than replaced him.
I am a sucker for Kate (the impeccable Sasha Alexander, the only actor that made Rizzoli and Isles watchable). Then she is replaced by the exotic Ziva (Cote de Pablo). Those two along with Dinozzo (Michael Weatherly) are the gold standard and I am torn.
McGee is a stalwart but only added to the cast as the probationary agent for an extended period. Once he became senior, he was done.
Abby and Casey are interchangeable as quirky, although Abby adds some sexuality to the role.
I do like Torres (Wilmer Valderrama) in the later seasons but the female agents after Ziva are forgettable with the small exception of the sexual tension between Torres and the quirky Bishop (Emily Wickersham). This was also the era of Clayton Reeves (Duane Henry) which was a weird dynamic.
Overall, it has to be Gibbs, Ziva, Dinozzo, McGee, and Abby. And of course, the impeccable David McCallum as Ducky and the beautiful Lauren Hutton as Director Shepard.
Sorry, Sasha.