Actually I found out that given enough training almost all pokemon in Diamond learn very strong moves later on and stronger ones earlier too.
If you're worried about types then I suggest catching one of each:
Grass - preferably Budew who evolves into Roserade
Flying - If you loved Pidgeot you're going to love starly
Ground/Steel - Almost anything is good except some half rock types. I don't like rock types for their weakness to like everything.
Water - A must, get Gyarados who's a monster in this game or consider Shellos
Electric - I got Shinx but if you're looking for quick power get Pikachu
Psychic/Dark/Ghost - Always good backup for your team. I highly recommend Sneasel for this.
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