Bradman has already been mentioned. Twice as good as any of his era.
Tendulkar, Lara, Ponting etc might be nearly equal to him in skill but they have had to compete at a higher level than he did back in the 30s and 40s.
Kipketer's 800m record is getting old, the only 2 runners close to him are Coe and Cruz from the 80s. El G's 1500 and mile times are well beyond any of the current top guys on their recent form.
Komen's 3000m remains well out of reach. Bekele's 5 and 10 are also well out of reach at the moment.
Flo Jo's 100 looks pretty safe for the next few decades unless some breakthrough occurs. Her 200 will stretch them for a long time too.
Koch's 400 time looks way beyond any current runner, I don't think anyone got within 2 secs of it last season.