This is a little late, but it's detailed and I spent a lot of time on it, so I'll post it anyways.
There are three factors: Teammates, Tiering, and Mega Evolutions
Dragalge benefits from a team that can clear out opposing Dragons, slow the opposition via Sticky Web and Thunder Wave, Dark-types to switch into Psyshocks and Levitators and/or Flying-types to switch into EQuake.
Dragalge, being an RU Pokémon, will really struggle in the OU environment. He will have to contend with all of the devastating anti-Dragon 'Mons that run rampant throughout the tier, namely Latios and Garchomp, as well as every Psyshock user ever. Even with Dragalge's decent Special Defence and Adaptability-boosted STABs, it simply cannot hold it's weight agains some of the more prominent threats in OU.
252+ SpA Choice Specs Dragalge Draco Meteor vs. 4 HP / 252 SpD Eviolite Chansey: 145-172 (22.5 - 26.7%) -- guaranteed 4HKO after Stealth Rock (not counting the repeated SpA drops – ouch)
252 SpA Life Orb Latios Psyshock vs. 200 HP / 0 Def Dragalge: 374-439 (116.5 - 136.7%) -- guaranteed OHKO
252 SpA Life Orb Latios Draco Meteor vs. 200 HP / 0 SpD Dragalge: 463-549 (144.2 - 171%) -- guaranteed OHKO
252+ SpA Choice Specs Adaptability Dragalge Draco Meteor vs. 4 HP / 0 SpD Latios: 704-832 (233.1 - 275.4%) -- guaranteed OHKO
Even though Dragalge can one-shot Latios with Draco Meteor, Latios outspeeds and one-shots Dragalge. It is also completely walled by Chansey/Blissey and useless against Excadrill. While Dragalge is usable in RU, it falls utterly short of the mark in OU due to the combination of superior Dragons, poor Defence and weakness to two of the most common moves in the tier, Psyshock and Earthquake.
Nothing to say here as Dragalge has no Mega Evolution.
Garchomp pairs well with bulky Water-types to absorb Ice attacks and bulky Steel-types to absorb Fairy attacks. In doubles, 'Chomp loves Levitators and Flying-types so it can spam EQuake and wreck things. (On a side note, the 11-0 doubles sweep is very gimmicky and is easily countered by priority Taunt, among other things.)
Garchomp has a nice little nest at the top of OU, and he's not likely to leave. His impressive HP, Attack and Speed allow him to perform a plethora of jobs. 'Chomp has a scary surprise factor, as you never know what set he's running until he's running over you. Garchomp's possibilities include ScarfChomp, LO, Lum Berry, Sashed SR, Mixed, and the odd support set, to name a few.
252+ Atk Garchomp Outrage vs. 4 HP / 252+ Def Eviolite Chansey: 286-337 (44.5 - 52.4%) -- guaranteed 2HKO after Stealth Rock
4 SpA Garchomp Fire Blast vs. 252 HP / 16 SpD Skarmory: 174-206 (52 - 61.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252+ Atk Garchomp Crunch vs. 252 HP / 252+ Def Slowbro: 260-306 (65.9 - 77.6%) -- guaranteed 2HKO after Leftovers recovery
While Garchomp isn't perfect, he has the raw power, good looks and versatility to have a spot on almost any team.
Mega Garchomp is meh. Upon Mega Evolving it gains better defences, decent Special Attack and sky-high Attack, but loses Speed, forcing it to run a bulkier set that regular Garchomp. It's Ability, Sand Force, gives it a nice niche on Sand teams and partners well with Tyranitar, but outside of Sand it isn't useful at all. Despite MegaChomp's buffed Attack, more often than not basic Garchomp actually hits harder.
252 SpA Garchomp Fire Blast vs. 252 HP / 168 SpD Ferrothorn: 268-316 (76.1 - 89.7%)
252 Atk Mega Garchomp Earthquake vs. 252 HP / 88+ Def Ferrothorn: 135-159 (38.3 - 45.1%)
As these calcs have shown, when choosing between Garchomp and its Mega form, there are usually better choices for a Mega and normal Garchomp performs better.
Salamence appreciates many of the same teammates as Garchomp, but also requires a Spinner/Defogger to clear out Rocks to allow for repeated switch-ins. Sticky Web is also useful because it patches up Salamence's average Speed.
Salamence has had always been at the top of OU, and has even been banned to Ubers a couple of times. But all good things come to an end, and Salamence was majorly nerfed in Gen VI, thanks to the introduction of Fairy-types. Suddenly Salamence had to contend with new threats like Sylveon and Xerneas as well as returning nobody-Pokémon back for revenge like Gardevoir, Mega Gardevoir, Mega Mawile (before it was banned to Ubers), and the evil Azumarill. Salamence didn't have the firepower or speed to muscle through these new threats, and it's only coverage move, Iron Head, proved inadequate to deal with Fairies.
252 Atk Life Orb Salamence Iron Head vs. 92 HP / 0 Def Azumarill: 140-166 (38.4 - 45.6%) -- guaranteed 3HKO
252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 0 Def Salamence: 432-510 (130.5 - 154%) -- guaranteed OHKO (without Belly Drum)
252+ Atk Huge Power Mega Mawile Play Rough vs. 0 HP / 0 Def Salamence: 668-788 (201.8 - 238%) -- guaranteed OHKO
4 SpA Life Orb Salamence Fire Blast vs. 172 HP / 0 SpD Mega Mawile: 234-278 (82.3 - 97.8%) -- guaranteed 2HKO
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. 0 HP / 0 SpD Salamence: 542-642 (163.7 - 193.9%) -- guaranteed OHKO
252 Atk Life Orb Salamence Iron Head vs. 0 HP / 24 Def Mega Gardevoir: 322-380 (116.2 - 137.1%) -- guaranteed OHKO
Although Salamence can OHKO Mega Gardevoir, it is outsped and KOed before it can get its feet off the ground.
Salamence's fall from glory was amended with the release of its new Mega Evolution in ORAS. It now has a beastly 700 BST and one of the most useful Abilities in the game, Aerialate. Aerialate managed to resurrect Pinsir in XY, and has worked its magic again in ORAS. Finally Salamence had a reliable Flying STAB in Return/Hyper Voice, and incredible bulk and speed to boot. It was so powerful that MegaMence was banned to Ubers, where it struggled to hold up against the horror of Mega Rayquaza, until it was banned from Ubers and MegaMence took its place as top dog. Take a look at these calcs:
Unfortunately, Smogon has not added ORAS Megas to its Battle Calculator (but MegaMence hits like a truck anyways).
MegaMence topples some of the bulkiest Pokés in Ubers, outspeeds the vast majority of unscarfed base 90s, and can tank some of the strongest attacks in the metagame.
I hope this overview of each Dragon helps you to choose one. Happy teambuilding!