The BEST will be VERY EXPENSIVE. You can get to 400 mm a few ways without big bucks but above that it gets expensive in Canon brand lenses. Popular choices are the 100-400 L IS, 400 f5.6 L & 300 f4 L IS plus a 1.4 teleconverter. You do however have a couple of other choices from Sigma that are longer (500 mm) but they have slower AF mechanisms.
"A skill is developed through constant practice with a passion to improve, not bought."
