Can't Chansey be caught outside of the Safari Zone as well? Correct me if I'm wrong, but I seem to remember you can get it in the Unknown Dungeon in R/B, or Y, or both, I can't remember.
So...for R/B/Y the Safari Zone does indeed hold the hardest Pokemon to capture:
Tauros: He's way more common in Yellow, than in R/B, so I wouldn't give him the hardest award.
Kangaskan: I seem to run into these all the time, not sure how their hardness to catch would compare, though.
Scyther: I got one pretty easily in Fr, and I doubt there's any difficulty difference between R anf Fr.
Pinsir: Haven't played Blue in ages. >< Last time I did, though, I just saved up the money and bought it with coins. The way I see it, if there's an alternate way to get a Pokemon, it doesn't count on hardness.
And in my original Red file, I just did the Safari/Seafoam trick, so none of the Safari Zone Pokemon gave me much trouble. :P
G/S/C: Raikou gave me more trouble than Entei--I actually got Entei down to 1 HP and asleep and finally snagged him with a Heavy Ball. Raikou I also got to 1 HP, but I had to use the Master Ball.
And one thing, before R/S/E came around, Level 70 Pokemon were insanely hard to capture. So I give Lugia and Ho-oh some credit as well.
~Chibi~