Which one you like? It can be design or abilities you get from the mega evolution.
Here my list
Mega Venusaur - Has a flower on it head -_- but thick fat helps with deal with fire and ice attacks \(^__^)/
Mega Charizard X/Y - Both have cool designs, Y with the added wings on the arms and extra horn and X with the new colors and blue flames. Y Drought comes in handy for a boost to fire attacks while X Tough Claws increase physical attacks.
Mega Blastoise - The 3 cannon set up is kinda weird. Mega Launcher isn't very useful since water pulse isn't great.
Mega Alakazam - Super beard and multiple spoons. Trace comes in handy in battles with trainers by getting the opponent's abilities.
Mega Gengar - A mix of Hunter and Gengar with a yellow dot on the forehead. Shadow Tag good to keep a opponent's pokemon from leaving.
Mega Kangaskhan - The baby Kangaskhan just joins the fight and allows for double hits.
Mega Pinsir - Super freaky. Aerilate is cool but my Pinsir mostly knows fighting and bug moves.
Mega Gyarados - Design reminds me of those Japanese fish paintings. Mold Breaker always welcome.
Mega Aerodactyl - Extra horns make it look like the devil. Tough claws benefits it stats.
Mega Mewtwo X/Y X looking like it came from DBZ. Same goes with Y to a degree but looks a lot simpler. Steadfast isn't great mostly because it doesn't happen a lot. Same goes with Insomnia, we're more likely to be attack than throwing out status effects. Mewtwo is already SA heavy so X form isn't very benefiting while Y really does.
Mega Ampharos - Grow hair, well visible hair. Mold Breaker always welcome being Dragon is cool.
Mega Scizor - Looks real awesome with the bulky claws and the blades under them. The V shape head and pointy legs makes for a sharp design. Technician pretty good if you do have moves that are at 60 and below (Metal claw is the only one I kept).
Mega Heracross - Super bulky, making the design a little too big than it should. Skill Link makes those multi hit moves better.
Mega Houndoom - The devil! Solar Power SA goes but but HP goes down D:
Mega Tyranitar - Has a face on it belly! Summons sandstorm, boo!
Mega Blaziken - Really cool kung-fu designs. Super fast.
Mega Gardevoir - So lady like with the big dress like legs. Fairy moves are pretty awesome.
Mega Mawile - It got not one but two mouths on it head! More attack stat!
Mega Aggron - Just got bigger. Filter reduced damage by super-effective moves so that cool. Unless it a SA.