I did like Cress some but not as much as I do now. As for why I picked Dialga, I've just always liked it's design and it's the reason I chose Diamond over Pearl. Dialga looked way cooler than Palkia imo.
The only trainer we've faced with a Legendary Pokemon is N. Should there be more trainers in the game's main storyline with Legendary Pokemon to battle against, such as Elite Fours or Champions?
It would be interesting to have more battles with other trainers using legendary pokemon. The game definitely needs to be a little more challenging and having to fight another trainer using a legend would be more of a challenge. But then you wouldn't be able to capture it.