It really depends on the gen, as it faces different opponents for each one. On the other hand, you've seemed to have been posting many gen 5 ou questions lately, so I'll go with that.
Enter Salamence. This is a Pokemon that was deemed too powerful for DPP OU, which is a testament to just how good it is. That doesn't explain everything about Salamence's power, though. In Black and White, the Dream World gods decided for whatever reason that this already powerful Pokemon needed even more muscle, so they gave it Moxie. Thanks to this ability, Salamence will progressively become more difficult to wall. So, are you thinking you can just send out a physical wall and expect all your Salamence problems to go away? Well, that's not how it works. Salamence has great mixed attacking stats, so you should probably think twice before switching that physical wall in. Okay, so, what does Salamence struggle with then? Answering that question is simple. It faces a lot of competition from Dragonite, who is much bulkier thanks to its stats and access to Multiscale. Additionally, the once unrivaled MixMence now has to compete with the newcomer Hydreigon, who boasts a higher Special Attack stat and access to Superpower. Salamence is also easily revenge killed by Mamoswine and Choice Scarf users such as Keldeo. Rest assured, however; Salamence is guaranteed to be a formidable opponent on the battlefield.
It's amazing, but it is also matched up closely by other dragons, drago unite and hydregon. It's also doomed against mamoswine(I should know. I use mamo on my gen 5 ou team) as an ice shard ohkos. It also gets koed by other Pokemon, like the mentioned keldeo.
Garchomp is one of the most versatile Pokemon in the OU metagame. Sporting great stat distribution across the board, an amazing typing, and above-average bulk, Garchomp is capable of filling many roles for a team. Garchomp's great Attack and Speed stats are augmented by powerful STAB moves, such as Outrage and Earthquake, that allow it to hit the majority of the tier for major damage with its STAB moves alone. While Ground / Dragon is a very good typing offensively, it cannot be disregarded defensively either. With an immunity to Electric, key resistances to Fire- and Rock-type moves, and above-average bulk, Garchomp is capable of checking strong attackers such as Thundurus-T, Tyranitar, and Heatran. However, while Garchomp has a good Speed stat, base 102 Speed just isn't what it used to be. Powerful threats such as Keldeo, Latios, and Terrakion are just some of the dangerous Pokemon that outspeed Garchomp. While Garchomp's Ground / Dragon typing is generally good defensively, it does remove its Water- and Grass-type resistances while only amplifying an Ice-type weakness that quite a few common Pokemon can exploit. Regardless, Garchomp is a very serious threat. It can begin to wreak havoc at a moment's notice, so be sure you have a way to handle it.
A bit more bulk, and with rough skin, it can be a bit more defensive. Not to be said that you should make it a wall tho. It's more on the fast-but-still-bulky-sweeper side, and it's typing is good offensively. It also is able to check Pokemon, which are mentioned above.
All in all, it really depends on the team. If you got yourself a spinner-maybe Salamence is better, no destroyed by SR. If you dont, maybe garchomp. Hyper offensive-salamence. Not hyper offensive-garchomp. Team to frail? Garchomp. Not enough damage? Salamence. It really depends on what the rest of your team is, as while they are similar they are still different.
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