Just to repeat a few things that are scattered through my exotic lens reviews and put them all in one place:
- Most Travel Friendly: (1) 200-400mm f/4, (2) 500mm f/4, (3) 400mm f/2.8, (4) 600mm f/4, in that order.
- Best optically: (1) 400mm f/2.8, (2) 500mm f/4, (3) 600mm f/4, (4) 200-400mm f/4.
- Hand-holdable: only the 200-400mm and 500mm, and only for short periods of time.
- Most Teleconverter Friendly: really only the 400mm f/2.8. All the others provoke immediate compromises even with the TC-14E.
- FX users: you often need more reach, so the 500mm and 600mm tend to get prioritized above the others (though watch the corners wide open).
A 300mm (either f/2.8 or f/4) is the fallback position if you need fast focus and reach. While the 300mm f/2.8 is optically equal to the 400mm f/2.8 (i.e. superb), I do not believe it to be the right lens for anyone unless 300mm is what you're going to use. The notion of buying 300mm and always using a teleconverter to get reach at a lower price and weight is a false economy in my book. In most cases you'd be better off with the 200-400mm f/4 than the 300mm f/2.8 with TC-14E.