Interest rates are pants at the mo, but without doubt, one of the best on offer at the mo is the Coventry - fixed 2.25% until Nov 2018.
It might seem a bit daunting committing for such a long period, as interest rates may rise soon (ish??), but if you want to close it early, the penalty is only 120 days interest, still making it highly competitive with everything else on offer.
The only downside is, it doesn't allow transfers in, but if you know you're not going to use up your £15k ISA allowance for the current tax year, you could simply withdraw the ISA and open up a new ISA with the money withdrawn from the old ISA ..... hope this all makes sense :)
More info here:
http://www.moneysavingexpert.com/savings/best-cash-isa
Scroll down to, "Top fixed-rate cash ISAs - even if you need access" (about half way down the page).