Latias-took me days to even find and then it ran away(I didn't know it would do that back then). Then it was really hard to find it again even with it's location in my pokedex. The darn thing moved so quickly!
Latios-same as Latias except it moved even more often than it
Feebas-I searched like avery single tile on that route and what do you know? It ends up being on the last tile on it!
Rayquaza-it took about 30 tries to catch that thing. I was so frustrated I wanted to throw the GBA SP out the window. Luckily I caught it right before I was about to(OK I wasn't really going to do that but I felt like it).