Optically, the 70-200mm f/2.8L IS (original) is the worst of the 5 70-200mm Canon zooms. The first and most important question is focal length - you mention extenders, so I presume you want more reach than 200mm on your 7D. Native focal length is always better than zoom + extender, i.e. the 100-400mm at 400mm beats a 70-200mm + 2x and the new 70-300mm at 300mm beats a 70-200mm + 1.4x at 280mm (although the new 70-200mm f/2.8L IS II + Mk III extenders comes pretty close). How much more reach do you need?


The next issue is aperture - the 70-200mm II + 1.4x III will give you 280mm f/4, whereas with either of the other lenses, you're at f/5.6 by then. Do you need that extra stop for shutter speed?


The third question is versatility. Do you need the flexibiilty of a zoom all the time? You'll get the best IQ in the telephoto range with a prime - the 300mm f/4L IS is one option, or the 400mm f/5.6L if you shoot with fast enought shutter speeds not to miss the IS feature.


The last question is budget. The truly 'best' lens would be a 200mm f/2L IS, 300mm f/2.8L IS II, or 400mm f/2.8L IS II (if you're not in a hurry, that is...the last two haven't been released as of yet, but the Mk I versions of those lenses are still better than any zoom, and take an extender without much impact on IQ).