best single year by a horse....

[miraja2]

For me the answer is:

Sunday Silence - '89.

I am biased because he was my all-time favorite horse but his year was truly amazing. For starters he won the two biggest races in the country - the BC Classic and the Kentucky Derby - in the same year. Unbridled is the only other horse to do that. The difference is that Unbridled didn't do much the rest of the year. Sunday Silence meanwhile won the G2 San Felipe and three other G1s - Preakness, Santa Anita Derby, and the Super Derby.
In addition Sunday Silence beat a true superstar of a horse in Easy Goer 3 out of 4 times and I believe EG was better than anything Cigar faced in 1995.
Don't get me wrong, Cigar was great in '95, but I would rank Sunday Silence's year #1.