IMO the WI would have the best ever ODI XI because they had the majority of excellent bowlers throughout the late 70's and throughout the 80's when ODI's had just started. Some of the bowlers that would be left out of the WI XI are classed as all time great bowlers. Many consider Joel Garner as the greatest ODI fast bowler of all time. The WI may lack a quality spinner but they make up for it with a fast bowling attack that would include the likes of Holding, Marshall, Ambrose, Walsh, Roberts, etc. The batsmen of the same period were at the peak of ODI cricket as well. The likes of Viv Richards (who would be IMO the greatest ODI batsmen of all time), Lloyd, Richardson, Haynes, Greenidge and in recent times Lara and Gayle are all top clas ODI batsmen. WI have an excellent keeper/batsman in Dujon.
I would place the Aussies second. Although they probably possess a weaker bowling lineup than the WI, they have variety. A great ODI spinner (Warne) and one of the greatest ODI bowlers (McGrath). They also have the hostility of Lillee and Lee and good part time bowling options (Bevan, Symonds). They have a fast scoring opening batting partnership (Gilchrist, Hayden/ M Waugh). They have a very strong middle order (Ponting and Jones). Jones was a very underated middle order ODI batsmen and both him and Ponting are capable of scoring hundreds. Australia also have Bevan who I would rate in the top three ODI batsmen of all time. He was able to adapt to any situation and often led Australia to victory. Michael Hussey is similar to Bevan and by the end of his career if he continues the way he is will probably be classed a better ODI batsmen. IMO they also havethe best ODI keeper/batmen in Gilchrist.
Both these teams dominated ODI cricket during their periods of glory, the WI during the late 70's and 80's and the Aussies from the mid 90's.
In no order:
The Pakistan XI would contain one of the greatest ODI bowling partnerhips (Wasim Akram and Waquar Younis) and also possess some excellent ODI batsmen (Miandad, Inzy).
The Indian XI contains some excellent ODI batsmen. IMO Tendulkar would be the second greatest ODI batsman of all time. Tendulkar along with Dravid, Ganguly and Azhar would make a very strong ODI batting lineup. They also have one of the greatest ODI allrounders (Kapil Dev).
South Africa have a team with some great allrounders. Klusener is one of the best lower order batsmen and capable of scoring at an extremely fast rate. Pollock is aquality ODI bowler and strong batmen with Kallis the opposite to Pollock. They also have one of the best ODI fast bowlers in Donald.
NZ possess a strong batting lineup (Crowe, Turner, Fleming, etc), excellent allrounders (Hadlee, Cairns) and a great spinner (Vettori) and fast bowler (Bond) along with good bowling support.
SL have a fast scoring opening partnership. Jayasuriya is one of the most explosive ODI batsmen of all time and a strong middle order (Sangakkara, Ranatunga, etc). They have an excellent fast bowler (Vaas) and superb ODI spinner (Murali).
England have strong batting which includes many explosive batsmen like Pieterson, Flintoff and Botham and two great allrounders (Botham and Flintoff). They also have a lot of good bowling options.