Radio tower by far....the first time I played crystal I had managed to defeat team rocket in mahogany town and then proceded to Blackthorn but that annoying guy in front of the den wouldn't let me pass and it took me ages to find what I was missing! Which I finally discovered completely by luck, when I returned to goldenrod for a pokemon in the day care..
Tin tower it took me a long time to get through it to get to Ho-Oh i always kept reaching a dead end and without a walkthrough it was really tough :o