On the evidence MS is least likely to be in the lead. Looking at overall sales going back to last gen they have shown good, steady growth but nothing stellar. Unless Natal really takes off then on current install base and penetration MS doesn't yet have the brand awareness not worldwide penetration to have a clear chance at number 1 spot (using consoles sold WW as the metric). They seem sure to continue to grow and build on current install base though, particularly in US/UK.
Sony has had two great generations historically, worldwide has a better brand still for games. However this gen so far they have taken a huge fall. They're slowly recovering, but still, a lot of damage has been done. Assuming they keep the momentum going with sorting out the issues they will start next gen in a not bad position but I think they will not see an easy marketplace to dominate. Short story, they need to recover there position with more general buyers vs a smaller hardcore if they want another shot and number 1 - there challenge is keeping the folks they got with PS3, combating MS in US/UK and knocking Nintendo from the slot they currently occupy.
Nintendo is in of course a great place - but there is cause for caution. Motion controls was a USP this gen vs Sony and MS and clearly drove their victory (so far). However next gen everyone will have that as everyone is going to finish this gen with it. The big question is, can they innovate that successfully twice in a row? The core Nintendo Mario audience isn't enough, as the Gamecube sales showed, they need the broader audience that last gen went primarily with the PS2. For Nintendo the challenge is bottling lightning twice while fighting off a competitor who's been champ twice and the new kid on the block who's been slowly but steadily gaining marketshare.
So, after all that, I figure number 1 next gen is most likely to be a battle between Sony and Nintendo depending on who produces the console to best capture the more general marketplace, with MS taking a bigger share of the pie yet again but still not gaining largest console install base market share.
However, with both Sony and MS motion controls to launch I think this question is now being asked too early. Let's reconsider this question January 2011 - with visibility to market reaction to Natal and the Wand vs Wii a lot more should be clear.
Final point, if Natal and the Wand to do well and start to balance out Wii USP, then next gen could really see a genuine 3 way split, with no majorly dominant console. That should make things really interesting.
Try to be reasonable... its easier than you think...