In Diamond it was for sure Gardenia, since I was a noob, I chose Piplup & I didn't know there were some ponyta in field next to the town, it took like 13 times to beat her. I literally had a mental breakdown by the 7th time.
As for Platinum, I'm still playing, but I think the water one will be really hard for me, since nobody on my team so far is good against water.