Not counting those that aren't currently available or the regionals
Three that come to mind for me are onix, aerodactyl and electrabuzz
I finally hatched onix and aerodactyl during the Easter event
Electrabuzz eluded me the longest, I only ever saw one on the nearby but I wasn't able to get to it. Finally with the most recent nest migration an electrabuzz nest formed about five minutes from where I work. I've only been able to catch two so far because it's not a real active nest, but I hope to catch a few more because they're decent for prestiging a gym.