First off, you won't be able to change Rotom Forms without the Secret Key, which is no longer obtainable without the use of cheats or a spoofed server to get the Mystery Gift.
Secondly, each of the Rotom Forms in Platinum are all Electric-Ghost in Generation 4. They also have the same 520 base stat total of each other (except base Rotom), which means the Rotom form I recommend is the one with your preferred move. Do you want Overheat? Use Heat. Do you want Blizzard? Use Frost. I think just about every form aside from maybe Fan is useful in their own way, so it's all personal preference.