Everybody has there favorites, but for general use I quite like BOC everyday E6013, or Murex E6013, avoid the cheap DIY unbranded supermarket ones, a mate bought some from N**to, for £3, a box, absolute waste of money, If you buy from a trade supplier by the carton (3 boxes of 250) expect to pay about £35 + vat.
As Paul has already said avoid the real cheap ones, you will never lay a weld with them, boc, murex, esab, or what I now use Hilco red extra, all E6013 general purpose rods. I would shop around but stick (excuse the pun) to the brand names. I get mine delivered from buck and hickman, don't know if they are the cheapest but my local supplier charges £26 a carton and I have to go fetch, whereas delivered from b and h costs me £36 for 2 cartons, a bit high compared to what tp has said so I may have to shop around a bit more.