Mega Garchomp is easily the worst Mega, or tied for worst with Obamasnow and Houndoom. It gets slower, losing battles with Manaphy with Ice Beam, Celebi w/ HP Ice, Anything else with base 100 Speed with HP Ice/Ice Beam, and other threats that don't come to mind at the moment but for sure exist, and the attack boosts aren't much needed, as you can kill most troublesome steel types in one hit when in Garchomp Normal form using a Fire Blast without much investment. Also, you lose your item.
The whole selling point for Garchomp is it's SPEED. SPEED! There is only one good thing about Mega Garchomp, and that is that you get Normal Garchomp's speed the same turn you Mega Evolve. But that's only one turn. It's not worth the item sacrifice in my opinion.
And now for you:
A LIST OF TOP SIX MEGAS IN MY OPINION:
1. Blaziken
All hail the king, baby. This thing is theoretically worse, because an LO Blaziken does more damage than Mega Blazkien. But oh well.
2. Kangaskan
Power-Up Punch is so freaking broken with Parental Bond. It's a damaging Swords Dance, for gosh sakes! It can tank a lot of hits, and it gets Sucker Punch too. Anything that is not Lucario (For Sucker Punch) or Sableye will die to this thing fast.
3. Gengar
Ok. LO Gengar does more damage than Mega-Gengar. This thing seems just like Mega-Alakazam. Doesn't seem too bad... Except it gets Shadow Tag. Single out your foes, and execute them! Shadow Tag is broken. NEXT!
4. Lucario
Adaptability Close Combat wrecks everything. Bullet Punch gets Adaptability boosts, and screws over any Ghost type outside of Golurk.
5. Mawile
Highest ability boosted attack in the entire game. Period. This thing's play rough can almost OHKO a full HP Dragonite with 252 +Def nature with Multiscale. That's scary. That's really scary. Also, it gets Sucker Punch, and access to Swords Dance, so we can wreck even MORE stuff! Not only that, but it has respectable defenses!
6. Pinsir
Aerialate + Quick Attack = Profit
After that, I'd be considering Heracross, Gardevoir, and yes, even Banette, which is deceptively good.