Creatively, it'd miss Microsoft the least. The XBoxes are the Michael Bay of consoles, with Microsoft's approach driven entirely by business (usually at the expense of the customer) and associating itself as much as possible with a very narrow field of middle of the road blockbuster entertainment. XBOX Live was a nice innovation, but entirely driven by its commercial potential.
For that same reason, Microsoft would be missed the most in commercial terms. Their total focus on the mainstream has expanded on Sony's success with the PlayStation and given the major publishers the safest bet they'll ever find in terms of making money, assuming they stick fairly closely to the accepted genre formulas. Yes, that approach has sucked a considerable amount of gameplay and design creativity from most major releases, but people love it regardless (I'm playing through Tomb Raider and increasingly despairing at those laudatory review scores) and at this stage, Microsoft can do whatever the hell they want and most likely get away with it. The 360 survived RRoD, charging for Live subscriptions, accounts being wantonly hacked and several other customer service controversies, but ask 90% of gamers what their favourite console of this generation was and Microsoft will unthinkingly get their vote. Industries need the mass-market juggernauts though, and Microsoft have done superbly to corner that market.
Nintendo are pretty much the opposite: I doubt they'd be commercially missed at all, but creatively, their constant experimenting with hardware design has inspired some of the industry's most important evolutions. Their implementation isn't always the best and their software brands are becoming increasingly reliant on formula in a bid to hold onto a fanbase alienated by the many hardware permutations and missteps (somewhat ironic, given how Microsoft have used formula entertainment to the opposite effect, for audience growth rather than consolidation), but when they do have a great idea, it tends to stick around, even if done better by someone else further down the line.
Sony, meanwhile, occupy a pretty neat middle ground, neither as outright commercial as Microsoft, but considerably more business-friendly than Nintendo and encouraging of smaller, riskier games like Journey and Ico/Shadows to accompany the AAA stuff and power-focused hardware.