I travel a lot by myself, usually 2-3 months every year and I've done 3 6-18 month stints of backpacking.
Best countries: Bolivia, Cuba, Canada, Peru, Argentina, New Zealand, Nepal, Botswana, Croatia, Italy, Iceland, Laos, Belize
Favourite cities: Havana, Vancouver, Cuzco
Worst: Dubai, Victoria Falls (the town, not the actual falls), Washington DC, Phuket, San Pedro Sula (Honduras) - I don't think I've ever been to an entire country I disliked, just these particular places