Personally I think 'best general of all time' is not quite accurate enough -
if you mean millitary commanders and success on the field then Napoleon, Hannible Heinz Gaderian, Alexander, and Wellington would be my top five.
If you are talking political longevity then Ceaser, Wellington, Cromwell, and Carl Gustav of Sweden would be my top four.
Another way of looking at it would be who has had the most impact on history - well Alexander carved out an empire that collapsed after he dies, Gaderian had no long lasting impact, Napoleon's empire also collapsed after his exile, Cromwell's hard work was undone after his death as well, likewise with Carl Gustav, so this leaves a toss up between Ceaser and Wellington.
The Roman empire in the west was build on Ceasers conquests, and the latin cultures in France and Spain can be attributed to his success. His legacy has lasted over 2000 years.
Wellington's victory at Waterloo enabled Old Europe to regroup, and avoid another continent wide war for 102 years (WWI) - he also enabled the UK to bring in reforms that averted a millitant revolution in this country (as he said it was 'a close run thing'). His vicotries on the continent enabled Britain to secure her empire overseas, and arguably enabled the modern world to develop as they have, with America and English the main cultural forces in the world.
Ultimately I would have to say that Ceaser in the best millitary commander of all time, purely based on his abillity to control both the army and the senate, and leave a lasting legacy.