Skarmory will wall Choice sets and Sub/SD (and still cause trouble to the SD/three attack set), not to mention sets up Spikes to wear down Garchomp in a way that Salamence and friends normally get worn down by SR. Bronzong can do so similarly, replacing Whirlwind/Spikes with the ability to hit hard using Gyro Ball and able to Explode.
The whole check/counter dichotomy these days is retarded anyway. Very few pokémon have any true "counter" by strict definition -- Infernape can predict the switch and U-Turn out of Cresselia! -- and how do you even "counter" a support pokémon? Make sure you're wasting moveslots on shitty Brick Break so you can eliminate screens and carry a Rapid Spinner and at least one weather user on every team?
And yet, even with all these other pokemon and all these ways to counter Garchomp, he's still more often than not a huge deciding factor in Suspect ladder games. HMM.
That's merely a function of 2/3 teams using it. Part of it is because, like Scizor, it's simply very easy to fit onto most teams. Garchomp's usage is also inflated because it's the most controversial Suspect and getting SEXP for it is important.
Maybe it would still be the #1 used pokémon in normal OU if it was let back in -- in fact, I'd put money down on that holding true -- but what does that matter? Scizor is #1 right now by over 50% to #2, Salamence, yet nobody thinks Scizor is uber and some are even clamoring for a Salamence suspect test. Usage stats don't tell us anything except what's being used.
DDMence has the same moveset as Chomp with the exception of DD/SD. Garchomp is far superior just because of the fact that it gets STAB on its 2 main attacking moves.
But where Garchomp gets STAB on its Earthquake, Salamence gets a much harder-hitting Fire Blast and the ability to blast through some physical walls with Draco Meteor. Garchomp can 2HKO Hippowdon at +2 with Earthquake or Outrage but Salamence can smack it for 90% straight-up on the special side, OHKO territory with even the slightest chip damage beforehand. Standard Garchomp barely does 50% to standard Skarmory with Fire Blast or +2 Fire Fang, it even needs SR to guarantee a 2HKO. MixMence can OHKO the same Skarmory (with
Flamethrower, without SR) and physical Salamence still deals significantly higher damage. So on and so forth.
Some people like to argue that Garchomp is better just because it doesn't
need to hit both sides of the spectrum. Who gives a shit? If I EV a pokémon offensively, I don't care if it gets 252 Atk, 252 SpA, 128 each or anything else in between. If it hits equally hard overall, it hits equally hard.
Salamence also has different resistances, making it able to switch into different things. Salamence tends to be a little less sturdy overall, due to its SR weakness, but it still switches in better against the types only it resists (especially Grass, 4x to Garchomp's 0x) and the Ground immunity is HUGE, huge like those capital letters.