Well the most important ones are:
VP$IP, PFR, 3b, fold to 3b
cbet flop, fold to cbet, c/r flop
aggression frequency, won at showdown


a large difference between VP$IP and PFR tells you they're loose-passive
a low VP$IP tells you're they're tight
a high PFR tells you they're aggressive

but that tells you nothing about how they play in 3b pots
some people 3b almost the same range they raise so they'll always call a 3b as well because their raising range is so tight it has a built-in call of a 3b in it
or some people almost always fold to 3b which means 3betting them with the goods is a waste of time since they'll never pay you off, you should bluff them with trash and flat when you have something until they adjust

you want to see if they cbet because if they don't it's pointless to try to check/raise the flop
at the same time you want to see if cbetting a pretty bad flop to cbet is going to be profitable since some players fold a lot to cbets and will fold an exploitable amount of time anyway
and some players just check-raise the shit out of you on the flop, so you should check behind at least some of the time

aggression frequency is just how aggressive someone is post-flop, it's a better measure than aggression factor
40%+ is aggressive
20% is passive

won at showdown tells you if the player goes too far with marginal hands or bluffs a lot
but it takes a long time to converge