The Gym Leaders are changed up a little bit in Platinum, so I'm going to do the same thing that the post did above me.
In Diamond/Pearl, it has to be Fantina. I really dislike the fact that she is so tough. I've lost against her quite a lot when I was on my journey. I was able to over level just a little bit, and she could still beat me. Then again, the Pokemon that I would use don't fully evolve until a few levels later. She once took out almost all of my team, except an Eevee. Some of her Pokemon's moves couldn't hit it, so it started to use Struggle and it died. I'm pretty sure that was my 3rd time playing Pokemon Diamond. It's a good thing that they made her a little bit weaker in Platinum.
In Pokemon Platinum, I think that the toughest Gym Leader was Crasher Wake. I have no clue why, I was able to beat him fine in Pokemon Diamond/Pearl but in Pokemon Platinum, I had no luck. It must have been the Pokemon that I chose in that game. His Gyarados would destroy a few Pokemon on my team, which wasn't much of a bother, but he took out the Roserade that I used. Then it all went downhill, the rest of my Pokemon would struggle against his Floatzel, that thing was fast. Roserade would be faster if it wasn't knocked out. If only I was smarter back then, I would have took a Jolteon to his gym and beat the Gyarados with it.
That's enough of my rambling for one day.