My lens lineup still shortage a telephoto lens. My use for this mainly for wildlife during hiking and jungle trekking. I'm thinking of below few option:
1) 200mm f/2.8 L with teleconverter. Cheapest and lightest but no IS, no weather sealing.
2) 70-300mm L. Most Compact and versatile. Reasonable price but only f5.6 at 300mm.
3) 300mm f/4 L. The best telephoto prime I can afford. Lose versatility of zoom but gain one stop faster at 300mm.
4) 100-400mm L. Most versatile but heavy and expensive.
5) 400mm f5.6. Cheap but no IS. I will have FL gap between 101-399mm.
All above 5 lens have very good IQ for me, my concern is which one is more ease of use when in the field.