Right now the biggest staple GX is Tapu Lele GX...which you might be able to tell by its price tag if you shop for it online. Its ability allows you to search for any supporter in your deck, so it's become a staple in any deck that benefits from Brigette.
Outside of that, some of the top performing GX Pokemon right now would be Gardevoir GX, Golisopod GX, Metagross GX, Drampa GX, Ho-Oh GX, and Alolan Ninetales GX. There are a handful of other GX Pokemon as well, but fundamentally speaking each GX Pokemon is a deck of their own. You don't just throw together a deck full of GXs and hope to do well. Since Pokemon have specific energy requirements in order to attack, you want to keep themes as thematic as possible. If you play Gardevoir, you want to focus on fairy energy and Pokemon that need fairy or colorless energy in order to attack. A lot of times, most decks won't run more than 1 GX attacker, at I'd say at max 3 different kinds of GX Pokemon. There is still a lot of value in having non-GX Pokemon both for having an attacker that only loses 1 prize when KO'd (GX Pokemon lose you 2 prizes per KO), and also a lot of non-GX Pokemon have very good abilities, such as Garbodor's Garbotoxin and Octillery's Abyssal Hand that can be played in accomodation with a lot of GX Pokemon.
If you're mostly interested in attack and defense, I would group the more popular GX Pokemon like so:
Attack focused: Gardevoir GX, Ho-Oh GX, Drampa GX. Gardevoir GX has an "unlimited" damage ceiling with infinite force, and people who play her are looking for back to back OHKOs. Ho-Oh GX can accelerate very quickly with Kiawe and swings for 180 base damage that can be modified by Choice Band, Fighting Fury Belt, Lurantis, and Volcanion EX to swing for OHKOs on everything relevant. Drampa GX hits for 150 on an optimal Berserk, Choice Banded for 180 damage which is one of the most important numbers to hit in the game right now.
Hybrid: Golisopod GX, Alolan Ninetales GX. Golisopod GX has conditional 120 damage and Armor Press, which allows it to take 20 less damage on your opponent's upcoming turn. It's very strategic, but the fact that it can hit for 120 damage for just 1 Grass energy is amazing. The GX attack is also a great finisher move that can also OHKO with choice band. Alolan Ninetales GX is a sniper that can deal 50 damage to any Pokemon on your opponent's side of the field, including the bench. It then has a fairly expensive, but powerful attack in the form of Blizzard Edge which deals 160 base damage and requires you to discard 2 energy of any kind. Alolan Ninetales GX likes to spread damage with Ice Blade, then Guzma up the bench for well timed Blizzard Edge KOs. Her GX attack is also amazing.
Defense focused: Metagross GX. There's probably no deck tankier than Metagross GX as it is a deck that benefits the most from Max Potion. Its ability Geotech System allows you to attach discarded energy to your Metagross GX, so even if you discard all of your energy from a Max Potion, you can just simply Geotech System it all back on and be at full health again, and with 250 HP, it's pretty much never going down unless its an OHKO.
You may also want to look into the new GX Pokemon coming out in Shining Legends and Crimson Invasion, as they have the potential to greatly alter how people construct their decks in the near future.