I wasn't going to answer but the other answer was about 5 years out of date (your tag is xy), I also included the three highest of each stat for both normal and mega Pokemon. Do note that Aegislash form changes have been taken into account. Source of info can be found here
HP: Blissey (255), Chansey (250) Wobbuffet (190)
ATK: Mega Heracross (185), Mega Garchomp (170), Mega Gallade and Mega Bannete (165)
Rampardos (165), Slaking (160), Aegislash (150)
DEF: Mega Steelix (230) and Mega Aggron (230), Mega Slowbro (180), Mega Metagross (150)
Shuckle (230), Steelix (200), Avalugg (184)
SATK: Mega Alakazam (175), Mega Gengar (170), Mega Gardevoir and Mega Ampharos (165)
Aegislash (150), Chandelure (145), Darmanitan Zen mode (140)
SDEF: No non-legendary mega Pokemon have a SDEF of >135 (Mega Gardevoir)
Shuckle (230), Florgess (154), Probopass, Aegislash, Goodra, Carbink (150)
Speed: Mega Alakazam and Mega Aereodactyl (150), Mega Beedrill and Mega Sceptile (145), Mega Loppuny and Mega Manectric (135)
Ninjask (160), Accelgor (145), Electrode (140)