For me, Clegg won.
Of course, he had it the easiest, as he was winning just by being there. All he needed to do was come across as credible, and the situation between the three parties would do the rest: fortunately for him, he did exactly that. Brown was substantive but not very personable, but there was little else he could say or do in his position; indeed, I'll go as far to say he did better than I expected. Cameron seemed uncomfortable and a little out-of-his-depth (almost boyish and lacking gravitas) when matters moved beyond strict scripting, but managed fairly well when he was regurgitating pre-prepared lines (which was best demonstrated by his closing lines). Brown was certainly the most authorative, and clearly a Prime Minister (for all he has his haters), but was out of his element, and kept falling back on Westminster jargon and combativeness to his detriment. Clegg, by comparison, was sure-footed, and was well prepared to address the people on their terms, not his, which helped him come across clearly. Cameron was obviously under orders not to be combative (unlike Brown), and that stilted his performance, especially as his policies were seemlingly lacking substance when pressured.
For me, the biggest disappointment was the lack of idological clarity between the parties, and the lack of clear message. They all sounded a little too similar, fighting as they were for the middle ground. More punch is needed to get through the general apathy. For example, compare Cameron's loose govern yourself slogans with the clear messages of Thatcher's day, such as 'I will empower you to buy your council houses!' Back in the 80s, that was a persuasive policy for many, and directly targetted a classically non-Conservative voter base with an obviously conservative policy (although, it didn't turn out well in practice); for all it may sound fainty prosaic today, so grounded are we in Thatcher's view of Britain, back then it was big news. Cameron (I'll single him out as he's the one trying to take power), has no policy with such obvious clarity of ideology and targetting that differentiates him from Labour. His position is all a bit vague and elusive, a bit not-Labour, and it is becoming increasingly apparent that he's exactly what he was originally touted as: the Conservative Tony Blair -- Tony Mark 2, with largely the same policies as Labour, but a bit more focussed on the rich that finance him. If you voted Tony, you're safe voting David! Not a surprise, for Tony Blair was an unmitigated success story for all his flaws, but I'd rather not have him back in power again.
Indeed, for me the most amusing part of the evening was Cameron's stongest showing was his eloquent discussion on the NHS. The NHS. A Conservative. It's almost a parody of politics. Certainly makes it fun to watch.
Anyway, round 1 to Clegg.
Clegg: Proved he deserved to be in the debates.
Brown: Uncomfortable outside of Westminster.
Cameron: Floundered when taken beyond his scripts.