None of the Legendaries are hard to catch if you have the right Pokes with you (Read: Wobbuffet), enough Full Restores, and enough Poke Balls.
Safari Zone Pokes are a whole different issue altogether. You get 30 crappy Safari Balls and a choice of either feeding the Poke (making it harder to catch), throwing a rock (making it quicker to flee), or taking a chance of doing nothing. Chansey, Kangaskhan, and Tauros are the most rare. Personally, I was able to catch 25% more Khans and Tauros than Chansey. Chansey is the most unpredictable and if you catch it on your first try then consider yourself very lucky. It took me longer to catch 30 of them than it did 30 of Khan/Tauros.