I just sided with Legion for this game(Mr House on previous) and arcade will never join me...this is ridiculous, how am I going to get the best armor of the game ??

Get him before you side with the legion?

you can't because doing his personnal quest only happens when you side with a faction. you HAVE to chose a side before getting gannon's quest. wiki.

Are you talking about the BOS power armor?

no, remnant power armor. the one that is rewarded at the end of Gannon personal quest

I've hear people talk about finding Remnants Armor in other places. I know for a fact there's a place you can get the helmet, though.

Deathclaw Promontory if you're brave and/or have a ton of stealthboys.