Aerodactyl, Arcanine, Cloyster, Donphan, Dragonite, Exeggutor, Gyarados, Heracross, Houndoom, Lanturn, Lapras, Mantine, Miltank, Piloswine, Pinsir, Rhyhorn, Skarmory, Snorlax, Stantler, Starmie, Tauros, Tentacruel, and Tyranitar are the hardest non-legendary FRLG Pokes to raise. They all take 1.25 million Exp Points to get to Lv100.
There are some RSE Pokes that are even harder: Breloom, Crawdaunt, Hariyama, Illumise, Seviper, Swalot, and Wailord all require 1.64 million. Now that is tough. Get one of them to Lv100 without Rare Candies or cheating and you have my respect. That is commitment.
edit: spelling