Here's the properties I expect out of a good player:

1. Able to correctly evaluate their own skill level. This is harder than it sounds, as shown by how many people think they're elite DPS until they run into someone with a parser. As someone who is nowhere near close to elite DPS, there's nothing wrong with that until you start acting like you're top dog and then need to be carried.
2. Carry their own weight for whatever content they're running. You're part of a team, at the end of the day, and good players don't tend to get carried (although we all have an off day sometimes). That means in levelling roulette you should be geared appropriately and active (looking at you, Shire tanks in Castrum Abania), in expert you should know your rotation and execute it reasonably, and if you show up in a savage two chest weekly clear party, then know what you're doing and be capable of savage level DPS.
3. Take feedback. If you can do #1 correctly, you should recognize when good advice is being offered. Listen to it, and evaluate it later. Don't get defensive, because usually people offering advice mean well.
4. Have situational awareness and put the group first. If four people including one of the healers just ate something and died, don't be the BLM that stands in stuff because "mah deeps!" A good player will realize that the one healer still standing is pretty busy and doesn't need someone making it worse by taking avoidable damage at a bad time.
5. Isn't a dick to the group. There's more than enough salt in alliance roulette already without adding more arguments, especially when people are already unhappy that the group managed to wipe in LotA. A good player will post a funny macro instead, or apologize if they caused the wipe (you'd be amazed how well that works), or even just keep quiet if they have nothing useful to add. Anything except adding more arguments. And for the love of everything holy, they won't be arguing with people in the middle of an encounter.
6. Use their kit appropriately. That means RDMs will Raise (or Cure if it's necessary because of a healer DC), healers will DPS when they can, tanks will use group defensive buffs like Divine Veil, etc.

You'll notice my list is pretty heavy on being a good team player, and very light on "must have cleared X with Y percentile". That's because "good" is relative. If the hardest content you ever want to run is expert, you do that competently, and you're fun to group with? Then you're good at what you're doing in my books.

Now if you're talking about who the top skilled players are? There's a website for that.