1981 Sebastian Coe
Indoor 800m wr, outdoor 800, 1000 wr (both 1.5+ seconds faster than ANYONE had ever ran, and lasted 15+ years each). 2xWR at the mile. Undefeated. Athlete of the year
1985 Steve Cram
WR 1500m, Mile (1 second off old record, stood for 8 years; 2000m, 1:42.88 800m beating Cruz. Not undefeated-lost a couple times at 800m.
1995 Nourredine Morceli
2000wr, 1500wr, world champion@1500, undefeated
(his '93 season of Wannemaker mile, world champion 1500 and 3:44 wr mile wasn't bad either)
1997 Wilson Kipketer
Incredible 2xIndoorWR (1:42.67), tied Coe's WR, then broke it twice by .62 total, World Champ. Athlete of the Year. Undefeated.
1997 Daniel Komen
2X WR at 2miles, wr at 5000m, world champion @5000m, 3:29, 3:46
1998 Haile Gebresalassie
Wr at 2000,3000m indoors, outdoor 5000+10000 wrs, undefeated
1999 Noah Ngeny
2nd to El G in wr mile and world champs, wr at 1000m...thats all really.
1999 Hicham El Guerrouj
WR mile, world champion, 2000m wr, 7:23 3000m, undefeated
2004 Kenenisa Bekele (tough guy to pick a best season)
Gold 10000, silver 5000, wr indoor 5000, outdoor 5000 and 10000 wr, double xc win
2005 Said Saeef Shaheen
world champion
2010 David Rudisha
2xWR by .1 total, ran 1:42.04, 1:41.51, 1:41.09, 1:41.01, 45.5 400m. African Champion, undefeated.
Your Call. (It's sure as hell not Shaheen or Ngeny!)