ITT: We make megas for the worst starters to make them at least usable. I'll start.

>Mega Meganium - Grass/Ground
Ability: Natural Cure

Base stats: 525 --> 625 total
-HP: 80 --> 80 (+0)
-Atk: 82 --> 110 (+26)
-Def: 100 --> 135 (+35)
-SpA: 83 --> 95 (+14)
-SpD: 100 --> 145 (+45)
-Spe: 80 --> 60 (-20)

>Mega Typhlosion - Fire
Ability: Sheer Force

Base stats: 534 --> 634 total
-HP: 78 --> 78 (+0)
-Atk: 84 --> 135 (+51)
-Def: 78 --> 75 (-3)
-SpA: 109 --> 135 (+26)
-SpD: 85 --> 85 (+0)
-Spe: 100 --> 126 (+26)

>Mega Feraligatr - Water/Fighting
Ability: Guts

Base stats: 530 --> 630 total
-HP: 85 --> 85 (+0)
-Atk: 105 --> 162 (+57)
-Def: 100 --> 110 (+10)
-SpA: 79 --> 65 (-14)
-SpD: 83 --> 110 (+27)
-Spe: 78 --> 98 (+20)