go with HP
252+ SpA Choice Specs Thundurus-T Thunderbolt vs. 252 HP / 4 SpD Mega Blastoise: 464-548 (128.1 - 151.3%) -- guaranteed OHKO
252+ SpA Choice Specs Thundurus-T Thunderbolt vs. 0 HP / 128 SpD Mega Blastoise: 416-492 (139.1 - 164.5%) -- guaranteed OHKO
252+ Atk Choice Band Electivire Wild Charge vs. 252 HP / 0 Def Mega Blastoise: 398-470 (109.9 - 129.8%) -- guaranteed OHKO
252+ Atk Choice Band Electivire Wild Charge vs. 0 HP / 126 Def Mega Blastoise: 356-422 (119 - 141.1%) -- guaranteed OHKO
so blastoise will take a little less damage With hp invested. therefore:
252 sp.atk/ 252 hp/ 4 sp.def
that would be the best.
hope I helped! :D