The bees that attack you when you shake a tree.I still havn't mastered that one.
Yeah! I find bees harder to catch than tarantulas/scorpions. Well I haven't actually caught a scorpion yet but I assume they move the same way as tarantulas. I can never get the timing right for bees... and I'll also add that I hate catching anything that falls out of a tree too because I'll shake it and not have my net ready, then fumble for too long through my letters to find my net and then they're gone :< I also used to think tarantulas were really hard to get but the only hard thing about it is not noticing them at first. My town has too many flowers so they can be really sneaky.
For fish, coelacanth. I'm never playing when they spawn/when the weather is right. ALSO ANY DEEP SEA CREATURE because I suck at diving.