I agree the Nokia 6300 is a great phone, however the battery life is questionable. I own both the 6300 and N95 8gb, and can talk for longer on the N95 8GB. That is just mathematics though, the N95 8gb has a 1200mAh battery.
If we are talking about ideal phones, with no respect to the size and weight limitations that obviously exist - which, by the way, is why something the size of a 6300 wont run s60 any time soon - then I'd want a modern updated k800i, thinner, running some super-tweaked version of s60, with the same great battery life and intuitive interface.
Alas, it was the perfect phone a few years ago, but it's now in a drawer. Let's have a moment of silence for the k800i.