My main six usually include a mix of Psychic and Dark. Sometimes Ghost and Dragon.
Gardevior is by far the best psychic with the Sp. stats. While Darkrai is the best Dark with the moves; Hypnosis, night mare, dark pulse, dream eater.
Gengar is my favourite Ghost type, while I choose the best dragon per game.
As for Mudkip, sadly I can only play him on Emerald :C but when I do, I never evolve him lol.
One tip for people who want to easy win against real opponents. Say you have a Dark pokemon, fighting is super effective against it right? Well, make your dark pokemon learn a psychic move, this not only allows it to have another move that can crush others, but it will destroy a fighting pokemon if an opponent sent one out without thinking you would do that. This not only works with Dark types, but every single type. Even the starter types. Like a Fire pokemon learning the grass move "Solar Flare".
And also don't judge, but when I got pokemon soul silver... I played it for 19 hour straight, got up to Victory Road too lol