Athletics, men's 10000m...that's difficult because a lot of great champions have run this race through last 100+ years and as you know, it's almost impossible to compare performances from so different periods...
I was really stuck in between Nurmi and Zatopek (even if Gebresilassie and Bekele also deserved to be mentioned), but finally I opted for Emil Zatopek because of his unmatched triple (5k, 10k, marathon) in a single edition of the Olympics.
Athletics, women's 10000m...I thought about Tulu for a while, but Dibaba was a more complete runner than her compatriot, so I ended up pickin' Tirunesh Dibaba...
Swimming, men's 100m Backstroke...maybe Roland Matthes deserves a better treatment, but he pays for sure the place where he comes from...therefore, I chose Aaron Peirsol, surely a better swimmer overall if we look beyond the single distance.
Gymnastics, women's Balance Beam...that's easy...despite all controversies, she's still unmatched, the true queen of this event... Nadia Comaneci.
Gymnastics, men's Horizontal Bar...modern athletes might have more technically difficult exercises and might be more exciting to watch, but I just have to pick Mitsuo Tsukahara, a man that left one of the most important footprints in the history of the sport (and the only one to win twice this specific discipline).