I have only just started taking notes on players, so relatively new to it.
I currently run a colour/traffic light system. Green is a player that I've noticed playing poor
hands, or someone who tries to
bluff far too regularly. Basically someone that should be easy to win from.
Orange is someone who makes some smart plays, and is typically a tighter player. Plays good hands and seems to know when best to fold etc.
Red players are players that seem to know exactly what they are doing, have much more experience than I do and seem to be making the right calls pretty much all the time. I am less likely to be winning money off of these people.
It's not at all a perfect system, and I have no doubt as I learn more I will adapt and expand it, but it's starting to work for me. I joined a cash game table and a number of the players were ones I'd tagged red. I didn't hang around and went to a different table.