Since I don't use a set team, the gym leaders' strategies aren't much of an issue. I rotate through pokemon constantly, and for gym battles, I put together a team specifically for that gym, so I always have multiple ways of dealing with something if one doesn't work (which got me through Tate and Liza fine). The only things that pose a problem are individual pokemon who are good enough on their own that they're a challenge even though I'm prepared for them. Of those, the toughest is probably Juan's Kingdra, followed closely by Wallace's Milotic, so that's my vote. Norman's Slaking isn't far behind though.