I went with Fantina, Candice and Volkner for the following reasons:
Fantina is one tough cookie, especially in Platinum (given she's now fought earlier on) and that powerful Mismagius of hers really annoyed me with its strong special-based attacks, which forced me to level grind like hell just so I could get to a sufficient level. I eventually defeated it by luck thanks to my Monferno scoring a critical hit with Flame Wheel.
Candice has an annoying strategy of abusing the hail. Her Pokemon can land 100% accurate Blizzards once her Abomasnow gets its Snow Warning ability going, also some of her Pokemon have Snow Cloak which makes it hard to land an attack when hailing. On top of that, she's tough to beat if you didn't start with Chimchar and don't have any Fire-types with you. Hardest Gym Leader ever in 4th Gen.
Volkner was tougher than I thought, but given he's the last Gym, it fits. His Pokemon are quick, and have strong attacks, and in D/P he didn't even have a mono-Electric team (he had an Octillery and Ambipom back then), he's easier to handle in Platinum if you have a Ground-type but still his Electivire's wide movepool can prove a tough challenge.