I'd say Morty, I've always really liked him even as a kid and as I got better at English it he ended up becoming one of my favorite Johto leaders.
Dunno about my favorite, but Bugsy and his freakin Scyther caused my first leave without save in my silver nuzlocke. Still havent got past him though, so I'm guessing this cant go well. Especially since I haven't reached Whitney and her dreaded Miltank everybody's talking about...
Yeah, if you're not ready (read: using a Geodude/Onix) that Scyther can really wreck your team (even moreso in HGSS due to that bug-type move U-Turn, which hits HARD off of his high ATK stat and protects him from counterattacks due to the auto-switch).
I'm gonna go with Pryce, I have a thing for gym leaders that actually seem like veterans who know what they're doing.
Also helps that his middle name is Willow, a name that is not insignificant in the Pokémon series, especially if you consider PoGo to be part of the Pokémon canon...