The big three:
Kunihiko Ikuhara (R 60-89, S, Supers series director)
S1: 6, 11, 15, 21, 26, 31, 36, 46
R: 51, 60, 61, 68
S: 92, 103, 110, 117
Supers: 128, 137, 150, 159, 166
[His episodes are always a step above the norm, especially in S1.
You can also notice trademarks of his that later show up in Utena and Penguin Drum.]
Junichi Satou (S1, R 47-59 series director)
S1: 1, 8, 14, 24
R: None
S: 90, 97, 104, 109
Supers: 132, 140, 146, 158
Stars: 172, 181, 198
[Satou's episodes are all pretty good, but especially after R.]
Takuya Igarashi (Stars series director)
S1: 24, 35, 42
R: 52, 65, 75, 88
S: 106, 115, 121, 124
Supers: 141, 151, 156, 164
Stars: 167, 173, 180, 187, 193, 200
[Not at the level of the previous two and his episodes can be kind of spotty, but he gets better as the show goes on.]
There are a few other directors worth taking note of, like Harume Kosaka, if I'm remembering right, but you also have some like Yuji Endo who is pretty much a workhorse director.