This will likely be unpopular but I am going to say summer 2012.
As horrific as that summer was, only the first half was exceptionally so, with a huge improvement from around the time the schools broke up. I remember a very welcome hot and sunny spell in late July, then an unsettled blip and finally a pleasant enough August. And whilst not technically summer I will mention it anyway seeing as some people consider it to be an extension of summer, early September that year was nice also. I can also recall a thunderstorm event at the end of June which did add some interest.
Still a poor summer, but 2007 was worse.