A few responses to aussie's picks...
For Lara, agreed on his test knock, but for his ODI one there's a few better I think. I remember two in particular, in consecutive matches in the 96/97 C&U series in Australia, where he scored a run a ball century in the first match and 90 in the second to get the West Indies home chasing big scores. The 90 was the better innings for me, because I remember watching it near the end and the Windies needed more than 12 an over, and they got it because Lara went absolutely mental and smashed about 50 off 3 or so overs with Samuels putting in some big hits too, including consecutive sixes off Warne and an over where he and Samuels absolutely destroyed Tom Moody. Finally off the last ball of the 49th over Warne got Lara out for 90 off 110, after he'd taken 80 or 90 balls to get to his 50. Bichel bowled the last over and got hit for four down the ground off the second ball and the Windies won, and Australia didn't make the final of their home tri-series for the first time in years. Amazing innings.
Hayden's 203 against India was amazing, and probably deserves to be his best, but his century against Pakistan in Sharjah in high 40s temperatures against Shoaib, Waqar, Saqlain and Kaneria was quite amazing too.
For Warne, it is amazingly difficult, but for me it's probably his spell against Pakistan at Brisbane in 1995. The most amazing thing about it was how Warne just stamped his authority on the series. Pakistan had just come to Australia, and at the time were considered a real threat to a team that were the best in the world but weren't exactly dominant yet. Pakistan had the two Ws at their best, Saqlain, and a fairly formiddable batting lineup including some excellent players of spin bowling. Steve Waugh scored a century as Australia made 400+ on a relatively flat wicket, and Warne came out an absolutely destroyed Pakistan, taking 7 for 23 in 16 overs and bowling Pakistan out for 97. Warne finished with 11 for the match and took 19 for the series as Australia won 2-1, but it was gone as a contest after Warne bowled that spell. Simply incredible.
Warne's best in ODIs is from the 99 WC semi, but people forget the 96 WC semi where he did the same thing against the West Indies and brought Australia back into the match from nowhere.
For Steve Waugh, I can't go past his 200 against the West indies in 95. You'd be hard pressed to find a better example of someone standing up against a brilliant bowling attack and defeating them. After Waugh made that famous 63 in the third test on a horrific pitch, it was so clear he had the edge over the Windies attack, and he came out in the next test and just mauled them. Amazing stuff.