Depends how you use the lens. If its for professional use no question 70-200mm F2.8 MKII. Brilliant lens, but its size and weight are not very desirable for travel. For a job the weight isn't an issue, travel it certainly is, It would be a tough pick but I would leave it at home specifically because of the weight. With your crop camera you are effectively getting a 110-320mm lens with F4 DOF equivalent. If you go down the 2x tele converter side you need another £350 and the lens AF speed decreases by 75% (which is well over exaggerated if you ask me). I used this combo at Silverstone Classics last weekend and it worked very well barely a shot missed and for me was easier than taking two lenses with IQ being similar to the 100-400. Price wise your looking at £2300 with the tele converter which is also a downside.
70-200mm F4 I understand why people buy the lens, smaller and lighter but at F4, but only for a FF camera. On a crop camera DOF equivalent is more like F6.3 wide open so your not gaining any added bokeh as subject isolation isn't brilliant, if anything I would always choose the F2.8. But on crop you are effectively getting a 110-320mm F6.3 DOF equivalent.
70-300mm is what I would be leaning toward, its small, light, great IQ and although similar to the 70-200mm F4 your DOF equivalent in FF terms is more like F6.3-F9 (obviously not in terms of speed but DOF) and you get an extra 100mm. In real world terms at 200mm the 70-200mm and 70-300mm are pretty much indistinguishable. The only disadvantage I see with this lens is the colour, when traveling the more inconspicuous the better and its a shame its white and it makes you stand out like a sore thumb. Its ok while on a job as people expect it but going into a country which is maybe a little less fortunate than your own with big bold flashy lenses is a disadvantage in my mind. But you are getting a 110-480mm F6.3-9 FF DOF equivalent, which is less than £900 its a compromise but a good one.
The 100-400mm is also a great lens, but it would be my last choice for a travel lens. Stupid push pull design, its huge and heavy, so unless you need the extra 100mm I would drop this off the list.
The only other option that has been missed off is the 70-300mm DO. I owned this lens but sold it because I needed a faster lens so bought the 70-200mm F2.8. Although this lens isn't perfect, its not an L lens, its not as sharp and has onion ring bokeh. For its size and weight is a brilliant travel companion, it is smaller than a 24-105mm and about the same weight. People dislike the bokeh and the slight halo you get but I found this is exaggerated, if your in a really high contrast situation it is there. Again its all about compromise with these zooms, the 70-300mm DO can be had for around £400 (second hand) which is more than half the price of the 100-400mm, 70-300mm and more than 1/4 the price of the 70-200mm. It is also black which is much more discrete and stop it down a little and it performs pretty well in the centre, corners aren't brilliant. Talking about bokeh, at DOF equivalent of F7.2-9 the bokeh you get will be minimal anyway similar to the 70-300mm L. So for £400 its worth a punt IMO but not at £1200.
My views above about the DO are strictly for travel only, professionally I would give it a miss. The thing with travel is that it is a compromise, you only get the shot with the equipment you have with you and if your traveling a good way weight is a really big issue, add a med body 800g a standard zoom 600g a small prime 300g and a super zoom 1000g the weight soon adds up and thats just camera gear.
I am in the middle of organising a 6 month trip across south america and the states, basically roughing it backpacking, so I will be carrying everything with me all the time.
My gear selection:
24-105mm F4 670g
35mm F2 IS 335g
70-300mm F4.5-5.6 DO 720g (or 70-300mm L 1050g not made up my mind)
Lots of cards & bats
2.6kg over 10 thousand miles is more than enough. Add a MBA clothes and supplies probably around 6-8kg.
1- 70-300mm L
2 - 70-300mm DO
3 - 70-200mm F2.8 + 2x
4 - 70-200mm F4
5 - 100-400mm