Non-Legendary: The best answer to this question is Electivire and its astonishing 123 base attack statistic along with a pool of move that matches this type of attack. He’s also quite massive, and can take a few hits , and is pretty quick for his size.
Legendary: This answer should not be a surprise to anyone, since it’s clearly being Zekrom. In addition to being a major legend his other famous competitions are Zapdos, Tapu Koko, Riakou, Rotom, Thundurus and Xurkitree. None of these legends are considered to be main (which is why their overall stats are considerably smaller). Along with the base stats Zekrom also masters the most effective Electric Type move Bolt Strike. The dual Dragon type puts him over other legendary players since the dragon type moves pool is absolutely crushing.
Mega-Evolution: Although not many electric types have Mega’s the ones that do are amazing. But the winner for this would easily be Mega-Ampharos. It’s special attack is literally the definition of god-like with 165 BASE stat. Although Mega-Ampharos isn’t very fast it is still a bulky pokemon, and wouldn’t get oneshotted in any battle with that 105 and 110 base defence stats. Along with all this he has a 90 base hp stat making him one of the most powerful mega’s.