I used to have this all planned out.
Water. Swampert, Starmie, Vaporeon, Lapras, Gyrados, and Cloyster, was I think what I wanted. Some are just cool, others - like Swampert - have types that complement each other, and others are just to make sure I have a wide variety of types within the water-main category.
Kyogre is also my favorite legendary.
I have always loved Steel types, Aggron and metagross are my bois.
Of course it would be Dragon! I would use the Lati twins, Hydreigon, Salamence, Haxorus and Garchomp. Dragons are my favourite theme no matter what world, I just love them so much. I do fear about Hydreigon’s future after introduction of fairies, though.