For me, it's pretty much all of the Safari Zones.
Generation 1's, while the best of the five IMO, still wasn't fun to navigate through, especially considering a full item run of the place would take at least 2-3 run through's of the zone, if you knew what you were looking for. Additionally, I can't stand searching and searching for all the rare Pokemon in this safari. As a kid, I was never able to catch Kangaskhan, Tauros, Chansey, Scyther, and Pinsir, making me very irritated towards this place for hoarding all the hard-to-get Pokemon.
'Generation 2's', aka the one in HGSS was a pretty fun