Honestly, I think Halo 4 was probably the easiest. I remember I first played it on solo normal the week it came out at my cousin's and beat it in like 6-7 hours. Then a few months later when I got the game myself, I beat it on solo legendary in like 5-6 hours.
I think the one I've had the most difficulty on beating on solo Legendary (excluding Halo Wars 1, which was a real pain to solo on Legendary) was Halo 3...gah, that High Charity mission where you had to deal with all the flood to rescue Cortana. For some reason, that took a little longer for me to do than I was expecting. Reach was also a bit of a challenge on Solo Legendary as well.
I still have to beat Halo: CE and Halo 2 on Solo Legendary so I can't so for certain which is still the hardest for me. Halo: CE is rather annoying on Legendary once you get to the Library, especially when you get to the points where you have to hold off waves of Flood while waiting for the Monitor to open the doors for you.