a couple of my favourites have to be the quiksilver and billabong logo's. Both brands seem to be so versatile in the use of their logo's and the typography that sits with them. They've changed and evolved their brand so many times over the years, yet it's always recognisable.
Not really a logo as such, but I've always thought that a yin yang is quite a powerful symbol and its representation of balance is reflected visually. There are a few companies which have used it an inspiration for their own logos too.
I love BP's logo. It is reminds me of the sun or the structure of a sunflower and points the way towards alternative forms of energy that all oil companies will eventually have to diversify into at some point before the oil runs out.
It's a shame that such a great iconic brand has become tarnished by the corporate irresponsibility that has led to the environmental disaster in the Gulf of Mexico.