Brock: Tough, lost to him thrice.
Misty: Kind of hell, had to get Charmeleon to level 30, teach him Mega Punch and get lucky in order to win.
Lt. Surge: Piece of cake
Erika: Even easier: Charizard made quick work of her.
Koga: Kind of tough, lost to him a couple times before Hitmonlee could get the job done.
Sabrina: Lost to her twice before Aerodactyl pwned her.
Blaine: Lapras. Surf. Easy one.
Giovanni: I made the stupid decision of challenging the Gym with Aerodactyl and it took me a whole lot of tries (his Rhydon always ended up KOing me. That was, till I taught him Toxic and spammed Fly like a madman.
And training in Diglett's Cave and the Cinnabar Mansion to get Lapras, Omanyte/Omastar and Aerodactyl to Lv 48 and catch up was also pretty tough.
But all in all, it was very challenging and fun!