At the risk of being repetitive, it all depends what you want the lens for. If you want to sit in a bird hide with a hefty tripod, then the longer the lens the better - the 500 is excellent and the 600 even better (especially if you have a porter to carry it) and it will beat out the smaller lenses with extenders - I'd love to have one of them with a 1.4 TC. If you want to stroll around with a relatively light lens with a good zoom range, the 100-400mm L IS is a great lens, provided you get a good copy. If you want to walk around with a camera slung over your shoulder or a heavier weight in your hand that is till manageable for opportunistic photos or sitting in a hide, then it is the 300. So, it's horses for courses. If you are rich enough, by them all. The 100-400mm is a good start to see what range you really want.