I find it amusing when Game Freak has you fight the same person multiple times over, yet their teams are strong early on and pathetic later on.
Giovanni in FR/LG is actually pretty tough the first time you fight him. He's slightly easier in Silph Co, and a total joke in Viridian Gym (like I legitimately never had trouble against that iteration of him, in contrast his FR/LG version actually gave me a headache at times).
Silver in GSC and HGSS is very challenging in Azalea Town, defeating me there many times over in Crystal. He's still annoying in Ecruteak, though not quite as difficult. Then you fight him in Goldenrod, and he's...eh. Fight him in Victory Road, and he's...nothing special at all.
Your rival in RSE counts too, not so much in ORAS given that their endgame team in ORAS is actually pretty strong. They are very challenging in that route (y'all know which one I mean, it's the second time you fight them IIRC), but beyond that point are moderately difficult at best.
I like characters that subvert this trend though. Blue more or less stays challenging every time you fight him, Wally becomes MUCH tougher, N too (albeit in his final fight), and Hau I think manages to stay relevant-ish from beginning to end, definitely a bit tougher in the end than at first glance.