I would have to say that the toughest is Elesa in BW2.
Any of them can be tough if you're not prepared. Among the more notable ones in that respect are Cheren and Lenora, since they both have pokemon with moves that will demolish an unprepared team (Cheren's Lillipup's Work Up and Tackle and Lenora's Watchog's Retaliate). But if you're prepared for them, they're not particularly tough at all. And that's the case with most of them. With Elesa though - even if you are prepared, she can still be tough. Volt Switch makes it tough to keep up with her pokemon unless you've got a ground in so that Volt Switch won't work, and if you've got a ground in, it's likely that it won't have a good attack against the Emolga, and it's certain that it will be so slow that it's going to take a lot of abuse along the way to finally winning.
I usually use a Trapinch against her, since it learns Bulldoze at level 21 and Rock Slide at level 25, so it can potentially handle her whole team, but it's so slow that it's touch and go anyway.
The only time I had a pretty easy time of it with her was when I went through once with a Dwebble and a Jolteon. Dwebble laid down Stealth Rock, then I switched in Jolteon, and it tanked the Volt Switches while her entire team beat themselves up against the Rocks.