Talking about battling terms, you have to go into movepool, trait, stats, etc. There are 4 different pokemon that are non-legendary with base stats adding up to the 600's - Tyranitar, Metagross, Dragonite and Salamence. The traits are all quite useful, except for perhaps Tyranitar and Dragonite. Tyranitar gets Sand Stream as a trait, which can be useful but will always hurt your team. It negates the leftover recovery on some pokemon, which some pokemon totally rely on for example Blissey, which will prove to be annoying on your team but will also make the opponent's physical and special walls easier to take down without the recovery. It helps on TSS teams (Toxic, Sandstorm, Spikes) but those do not work as well in 386 OU as they did in GSC.
Dragonite's ability, Inner Focus, stops it from flinching. Although useful against something like Salamence to stop it flinching you with a Rock Slide then killing you, it will not be that useful outside of Rock Slides - Intimidating pokemon so you can switch into them easier like Salamence has will prove to be more useful than withstanding a few flinches from Rock Slides.
This leaves Salamence and Metagross, both powerful CBers, powerful sweepers relying on raw physical attack to tear through teams. Metagross has access to the strongest Explosion in the game IIRC, and a whopping 405 attack to go along with that. It's only downside is it's speed, which it can ignore with agility. It's trait is very useful, Clear Body, which means it cannot be intimidated, speed lowered, or affected by any stat lowering move, which makes it easier to sweep so Intimidaters don't stop it. It has a good arsenal of moves, such as Explosion, a STAB Meteor Mash, Earthquake, Sludge Bomb, Shadow Ball, Rock Slide, Psychic for Weezing and Thunderpunch to even hit Skarmory. It has agility to raise speed as well, which makes it a great pokemon.
Salamence though, is more speedy. It can intimidate stuff like Heracross, switching into it's megahorn like nothing and kill it. It has stab flying moves, dragon dance, and other physical moves to make use of it's, just like Metagross', 405 attack. It's an ideal CBer with it's good speed and gigantic attack, and can dragon dance too with a great physical movepool. It has Aerial Ace, Rock Slide, Earthquake, Fire Blast, Hydro Pump, and other good moves. Both Metagross and Salamence are extremly good sweepers, Extremly good physical attackers that simply rip apart any sort of pokemon in their way.
Yet you have to admit, pokemon have counters. There are certain pokemon that have the capability of defeating another one, and even those get beaten by another one. There is no single pokemon out of all the 386, that cannot be beaten by something, that cannot be countered. Every pokemon has a weakness to another, and even if they're movepools are bad, they're stats are bad, they have the capability of countering something else, perhaps something in NU play. There are different metagames, and you can't judge pokemon for their battling ability in a different metagame in the one they are supposed to be in. Let's say, put a Tropius in OU. It won't do much, it won't damage much, and will get beaten very easily, with all the pokemon there superior in stats and movepool. But put it in NU play, and it does beat stuff, counter other pokemon and have a use. There is no best pokemon, as each are designed to do something else and none are invincible.