As a little kid on my first playthrough the hardest fights I found were:
RBY:
-Misty of course, that Bubblebeam....
-Gary on the S.S Anne
GSC:
-Whitney's Miltank
-Jasmine's Steelix, holy hell I had no idea how to kill Steel types I got massacred there
RSE:
-Norman's Slaking
-Liza and Tate
I...