The first time I played Sapphire (which was when I was like 5 or something) I would have a hard time fighting Wattson just because the only ground type I had was my Marshtomp and I didn't know anything about Pokemon at the time. It probably would've helped if the rest of my team wasn't all part flying type. (Taillow, Wingull, Beautifly)
But I think Norman was definitely the most evil. It took me about 8 tries to beat him, then I drop my Gameboy and the game falls out and I have to fight him like 4 more times. Everyone else wasn't too hard. Nothing I couldn't handle once I got a hang of things.