Dialga server is an fairly competitive server, If you are looking for a server where you'd have to grind a little for your achievements this server is your best choice.
Giratina server is fairly moderate level. This server brings in the best balance, for an excellent pixelmon experience.
Palkia server is for those who just started on their adventure on Pixelmon, This server is fairly easy with higher spawn rate of shinies and legendaries. Best server for the new trainers !