IMO, the only parties who deserve to win are the Lib Dems, UKIP or the Socialist Party. And I'mma tell you why;
Labour and the Conservatives don't have policies. They might claim to have policies, but let's look at the facts shall we? Labour has repeatedly lied about their manifesto, going on to sometimes do the exact opposite of what they've said. Cameron has effective turned the conservatives into Labour 15 years ago. The only difference is that Gordon Brown hasn't won an election. They've effectively turned the election into a popularity contest, counting more on people voting for Brown or Cameron than what the parties actually stand for.
The Greens and BNP, though I'm undoubtedly going to offend some environmentalists here, I don't overly care, are both fascist parties and it's the duty of the voter to prevent them getting seats in parliament. (I don't mean to say all environmentalists are fascists, but the Green party definitely is)
Which leaves us with the other 3 "big" parties.
The Lib Dems are the centre party, they've got my vote. You probably know who they are so I won't bore you with the details.
UKIP (United Kingdom Independence Party) are the only moderate Right Wing group left in the UK IMO (I don't think the Conservatives are really right wing anymore, they're more into appeasing everyone than actually having policies). I'd vote for them, though the constituency I'm in is divided between Labour and the Lib Dems, so... yeah...
The Socialist Party (aka "Socialist Alternative") is to Labour what UKIP is to Tories. They're Trotskyists, I don't pretend to understand every facet of Socialism, so I'll leave it at that. If you're planning on voting for a proper left-wing party you probably already know what Trotskyism is, if not, then you probably don't care. (s'all Marxism to me)
I haven't included Plaid Cymru, Sinn Féin (sorry if that comes out wrong, it's acting up in the preview box), DUP, UUP or the SNP, as they can't really form a majority government (AFAIK Plaid doesn't run for seats outside Wales, ditto for Sinn Féin, DUP or UUP and North Ireland and the SNP and Scotland). Not to say they don't deserve voting for, because that's definitely not the case.
The important thing to do is to find out who's running in your constituency and what they stand for! Persuade your friends to do the same! Ignorance of how the system over here works is the reason we haven't had the leaders we deserve for last 2 decades!
Not that I have any right to tell you who to vote for. By all means, vote Labour or Tory if you want another 5 years of crappy economy and kowtowing to the Yanks. (I mean, both they're slogans are effectively "Change". It worked for Obama because he's cool, Brown and Cameron are not cool. They are to cool what a penguin is to Alpha Predator-ism)