Is there one philosopher that you think is the greatest/wisest?
No. I do not believe a successful life can be lived by following a single rigid philosophy, code, or rule. Philosophers can tend to become a substitue for a priest or religion. When that happens it is bad.
Some 'traditional' philosophers I respect: (not in any particular order)
Lao Tzu - (Tao Te Ching) High return on investment per word.
A philosophical Taoist is something like a liberatarian environmentalist who rejects violence and aggression as a means to an end.

LT was robably not a real person though.
Nietzsche - By far, probably the most maligned and misunderstood philosphers. Every artist or person of a creative temprement should read Nietzsche.
Sun Tzu / Machiavelli - Yeah, learn from the dark side Luke.
Aristotle - Aristotle has a great way to understand how to develop character by practicing certain habits of life. Some Aristotle is just plain obsolete, but a lot of useful modern 'self-help' writings are based on Aristotle. (IMHO Covey's 7 habits borrows quite a lot from Aristotle.) A good way to understand Aristotle for
modern times is through the writings of Mortimer Adler. Can be found on the web in many places under the nomen 'The Great Ideas'. Yeah I like Adler, so sue me.
The sparks
really fly within
scientific philosophy. That's more difficult to pick a 'camp', so I generally don't. I like reading Sagan, Popper, Gardner, Einstein, etc.
There are 'asthetic' philosophers - writers, artists, and poets. There is much to learn. These would be people like Matso Basho, Picasso, or Escher.
humm.. not nearly a complete enough list.