Here is my opinion as a relative outsider to the specific people involved. I'll try to keep my bias to a minimum in my rankings.

1. Nicola Sturgeon: Absolutely stellar, the whole way through. I don't go in with her ideology at all, and disagree with her on Scotland. That said, she knocked it out of the park. She hammered home what Scots wanted to see without seeming like an anti-English taxpayer pirate. I was particularly impressed by her attacks on Miliband in terms of supporting the other English parties.
2. David Cameron: By a narrow margin. It wasn't spectacular, but the Tories didn't need spectacular since they didn't want the debates anyway. Given his past performance, quite a bit was at stake. I thought he came across as more steady than the others, and I also think he was less of a broken record about the economy than in the past. He emphasized the message of staying the course, but I think also performed best in the NHS portion.
3. Ed Miliband: Not quite his performance in the thing with Paxman, but he avoided major mistakes. I find Miliband to seem like a decent person, and he also managed to avoid most of the attacks from the left other than those from Sturgeon. I thought he was weak on the NHS, weak on immigration, but great on future for young people and not bad on deficit.
4. Leanne Wood: She didn't do great, but I think to Welsh voters she has to have impressed and also received quite a bit of publicity. And the attacks on Labour about the Welsh NHS can't be anything but effective, largely given that Welsh voters will notice that Ed Miliband contrived to ignore them and talk to David Cameron every time she brought it up. I also think her stand on immigration, which I completely oppose, was particularly brave, rising right to the challenge from Farage.
5. Nick Clegg: He presented himself as a middle ground for a coalition, and did a decent job. But it seems like his only point is on the economy and apprenticeships, which is already quite crowded. Were I English, I might vote for him, but that is my own bias. Cameron, Farage, and Miliband all seem to have a better idea of their bases (the Lib Dems don't have one), and the psyche of a modern English voter. So much for Cleggmania.
6. Nigel Farage: Nigel, Nigel. You performed alright, but you could have done great. As someone who enjoys watching him speak in the European Parliament and thinks he is the best speaker of them all, that was horribly disappointing. Turning every question to immigration will not increase your vote share. His best points aren't immigration and health tourism, they are the EU (in particular the ECHR) and housing. He failed to provide any kind of splash moments, and his only strong point was when the talk turned to the EU directly. Even then, the other party leaders were able to unite and bash him on immigration.
7. Natalie Bennet: Quite unimpressive, and unfortunately Cameron's plans to get her to take votes off Labour may make the public less likely to vote for her. Her points were against David Cameron, which was the first flaw. She isn't engaged in a battle for votes with him, she wants to beat Labour and take over the left, which means Miliband. Of the three farther left leaders, she took the least shots at Miliband, and ok this one might be a little biased, but Nick Clegg annihilated her on the NHS.

Winners of each Question

Deficit: Nick Clegg. The leaders who didn't take on the deficit just didn't figure in, and Farage was laughably pathetic with his "please notice me" act. That leaves Cameron and Miliband. They produced their usual rhetoric, Miliband about fairness, and Cameron about cuts and hard choices. Clegg cut the line between these two, and they found it difficult to attack that ground in the debate.

Immigration: Nigel Farage. This one was a given, and a bit of a pyrrhic victory anyway, because he tainted it all with his silly comments about health tourism. Regardless, given that he was the only anti-EU person in the debate, he was able to corner Cameron and fight everybody else off to stick to his message.

Health: David Cameron. He fought off Labour attacks about privatization, and Nick Clegg had to get in that side fight with Bennett. Miliband was looking particularly creepy in this section, and the other three leaders failed to bring anything relevant, with Farage being just horrible. Consolation victory to Leanne Wood who did her job in slamming Labour on the Welsh NHS.

Future: Nicola Sturgeon. This one was by a country mile. While Cameron and Clegg made petty argument about the coalition, she cornered Ed Miliband and took him to case for not being sufficiently left enough and not caring about Scotland, and took no prisoners with anyone else. Farage sounded weak as through the whole debate, Bennett seemed quite irrelevant, and Leanne Wood had very little interesting to say.