If Kanto Gym leaders in GSC count, Blue is pretty fun to fight, since his monsters are all diverse types and therefore more challenging, unlike other gym leaders. His 1337 attitude never changes...
As far as elemental gym leaders go, it would probably be Tate & Liza, since it is the only gym leader battle that is a double battle and is rather difficult. Takes more strategy than usual to defeat, especially with under-used monsters. I normally end up fighting them 1-2 times before sucessfully defeating them.