Actually, the longest-lasting battery I've seen was on my LG-LP3700. But I think you can only get and use this phone in Korea...
On the other hand, a new Nokia phone might take the cake. From their press release:
Nokia 1110i: latest addition to range of phones aimed at first time users
Nokia 1110i emphasizes ease-of-use, reliability and affordability and the intuitive, black and white user interface makes full use of graphical icons and large font sizes. The Nokia 1110i also has a number of cost management features, including support for the Nokia Prepaid Tracker to help people monitor their phone usage, as well as other technological features which reduce the total cost of mobile ownership for consumers.
Weighing 80 grams, the Nokia 1110i has a talk time of up to over 5 hours and a standby time of up to 15 days. The estimated retail price of Nokia 1110i, before subsidies or taxes, is expected to be ?45.