I love Team Plasma as a whole, and consider them as my favourite villainous team till date. However, the one who stood out to me the most was Ghetsis. He was the one who made me love Team Plasma so much. Yes I know N is the one who is the deep character from being raised in horrible circumstances and his undying love for Pokémon and willingness to fight for truth / ideals. However, N's troubles only further show what a huge scumbag Ghetsis was.
Ghetsis is definitely the most evil Pokémon villain in all the main series games so far, due to the manner he plotted everything. He didn't plan to rule Unova one find day, but rather carefully planned it for so long. He brings home an orphaned N, and moulds him to his bidding by showing him only abused Pokémon, so that N is convinced all humans abuse Pokémon. And not only that, he planned to get rid off N anyway, even if he successfully convinced all the trainers in Unova to release their Pokémon.
And in BW2, he's a madman who doesn't even want to deceive anyone anymore, and tries to rule Unova by force. And at the end of it all, he suffers a mental breakdown, yet, is never even remorseful for what he's done.
Ghetsis is not only hands down my favourite Team Plasma character, but he's my favourite Pokémon villain across all 7 gens.