DNA's CMT of Ruby has inspired me to do a CMT of my own on Pokemon Platinum. While it won't be nearly as entertaining as his, it will (hopefully) be somewhat entertains for you guys.

Anyways..
Da Rules

~ My team will contain at least one each of Grass, Water, Fire, and Flying
~ My team will be randomly drawn from the suggestions made, after filling the above requirement
~ I will be training 8 Pokemon, for the sake of flexibility for gyms
~ I will reject any suggestions that I feel to be unreasonable
~ Piplup will be my starter unless other suggestions are made

Have at it fellow Psypokians! Give me a team to beat the gyms, destroy the league, and take over the world Sinnoh!

Pokemon
Rapidash
Leafeon

Okay great!

Here's my suggestion...

: This pokemon alone can get through the whole game, already proven by me. It has great stats, and Sunny Day + Solarbeam will take care of its 3 weaknesses: water, rock and ground. Man, dat powerhouse.

Might think of something else also.

I've never actually used Rapidash before...
Definently looks like something I'd use though!

In my last Platinum playthrough I had a lot of fun Swords Dance + Leaf Blade-ing my way through everything with Leafeon, so that's my suggestion.

Well I seem to be late to the party but I figured I would go ahead and throw my two cents in. I see you have Piplup as your starter and your Plant and Fire type covered, but I would still suggest Gyarados. They are easy to obtain a good nature on seeing as Magikarp are abundant early and a solid Gyarados can sweep an entire team with ease. His 4x weakness to electricity can be a hazard, however you could use it to your advantage with other pokemon you could switch in to. He also takes care of your need for Waterfall.