What about Tyranitar being considered an uber? It has a total base status of 600, much like many Legendary pokemon, but it technically has more considering the special defense boost given by sand stream, which makes it have more than Garchomp which has 600 base stats. Also, Garchomp and Tyranitar, along with Dragonite, Salamence, and Metagross, are powerful and all five have base stats of 600. User:Yoshi Egg
It's much faster (Garchomp has 102 Base Speed, while Tyranitar has Base 61), and although it has 4 less Base Attack (not much of a difference), it can kill very quickly. And you won't need too much Def. or Sp. Def. if you can defeat the opponent before they defeat you.
Tyranitar can't do much to faster Fighting-types.
It is the issue of Speed here. Both Garchomp and Tyranitar are targeted by their weaknesses, Garchomp's being 4x to Ice and Tyranitar's being 4x to Fighting. Garchomp can easily survive one Ice attack from a faster Pokemon with a Yache Berry before sweeping a team, whereas if Tyanitar survives a Fighting attack with a Chople Berry, with only Base 61 Speed it's unlikely to outspeed the next major threat. Whilst Tyranitar can increase its Speed with Dragon Dance or Rock Polish, Garchomp doesn't need the Speed boost and can spend its time increasing its already formidable Attack with Swords Dance. Bear in mind that after a Swords Dance Garchomp can outspeed AND at least 2HKO a heck of a lot of Pokemon (in fact, I think it 2HKO's nearly every single Pokemon) in the game.
Garchomp also boasts much better type coverage with its STAB attacks, with Dragon and Ground being unresisted by every Pokemon in the game except Skarmory, Bronzong and Shedinja, all three of which die to Fire Fang. That and the power of its STAB attacks (140 for Outrage, 100 for Earthquake) is huge compared with Tyranitar's.
Also, whilst not something to be explicitly considered, Garchomp has Sand Veil, boosting its Evasion by 20% (or lowering the opponent's Accuracy to 83.3%, whichever you prefer sounds more impressive). Imagine you've gotten rid of its Yache Berry, you bring in something quick to kill it and you miss. Garchomp kills your team.
Whilst it is true that Tyranitar is more bulky, it has glaring weaknesses to two of the most common types in the game in Fighting and Ground. Combine this with its overall lack of Speed and the damage soon racks up; even with its defences and HP, it's not going to stick around for long enough to sweep if your opponent knows what they're doing. Garchomp on the other hand is a much bigger problem; keep it for the late game and it can be ridiculously easy to set up and sweep. Garchomp NEEDS two Pokemon to stop it every time; Tyranitar is stopped much more easily.
Ubers is reserved for Pokemon with no single counter in the standard metagame, and those with the potential to break the metagame. Garchomp seems to fit this category on paper, all that remains is to test it.
Garchomp is one Pokemon that can easily force an opponent into submission. It can 2HKO Pokemon which resist its attacks after a Swords Dance. It even 3HKOs Skarmory, an extremely defensive Steel-type, with Outrage. What's more is that it even 4HKOs a Steelix (Which has one of the highest defenses in the game) with Outrage if it maxes out its HP and defense (which it never does). Salamence, on the other hand, is weak to Stealth Rock, slower and easier to counter. Salamence is indeed powerful (and also has the element of surprise when it first appears), but I'd be leaning toward Garchomp due to the sheer force it has. Anyway, it really depends on the resistances that your team needs. Garchomp has a nice electric immunity and a good number of resists like to rock, while Salamence has a ground immunity and a resistance (plus Intimidate) to Fighting. Overall, it really depends on what your team could use more in terms of offense and resistances.
Anyway, Garchomp really is Uber material. Sure, it can be countered, but it will always require a sacrifice of one of your team members. Even after that, Yache Berry allows it to survive whatever Ice attack you just threw its way and KO another Pokemon. By playing it properly, 2 or more opposing Pokemon are guaranteed to fall. I'm mostly reiterating what Ajnidehs said, but its true.