Don't know about best but I can recommend the Post Office for the over 80s. Friend took out a policy for her 83 year old mother when she realised she wasn't covered just before we all went to Italy in Oct. We had a car crash, the two couples are still waiting for their ins companies to cough up, meanwhile the PO not only covered getting 83 year old home with nurse but have also already settled out of pocket expenses etc.
Go through confused.com. Post office was more expensive then most of the results. I cant remember who we insured with now but my mum who is 65 got platinum cover for �18 and my family of 4 was �16.
May be a bit late answering this but it could be useful for someone else finding it? I used http://www.insureforall.com/ for my parents as they specialise in senior citizen travel insurance. They also got nominated in the Insurer / Underwriter of the Year with another award so I assumed they were pretty decent!