Seriously or comedically?
Because I put quite a lot of thought and research into committing murder for a book I was writing. It has to be somewhere close enough that you can commute easily by method of easily accessed public transport, ie, train, bus, taxi, not a place you can be easily picked out of a lineup, which pretty much discounts taxi, and is far enough away to not be an obvious choice in a police search, which would also rule out the bus.
It would have to be a major city, preferably with a high rate of crime already, which for Britain would mean London, Manchester, Leeds, Birmingham (there is a list). The major city makes it harder to track an individual immediately before or after the murder, and also means that there would be more trains (as mentioned above) leaving more frequently, so it would make it a nigh impossible task. And the high rate of crime, particularly if you went to somewhere like London, would mean that there would be less interest in your particular murder.
Choose a group that no one cares about, who have murders happen all the time, which for the purposes of this argument would probably be a black teenager or young adult in the East End of London, kill them quietly when they're in an alley in the early hours of the morning. Between 2-4 am would probably be the best time, before the night shifts get off and the morning shifts wake up, but after the time most of the clubs have turned out. It has to be utterly random, with no time spent leaving distinguishing marks or messages, while you wear gloves and some sort of cover-all suit.
Remove the suit and ball it up into a bag, the murder weapon (best if it's a knife) goes in the Thames, the bad and suit need to be burned, and again London has lots of nice quiet places you could do this, hop on the first train out, and bam, perfect murder, you're not even remotely a suspect, because you don't even live in the same city.
Comedically, I'd kill someone through the internet, just so Christopher Walken could investigate my case. (If anyone gets that reference they get a video)