The best I have used personally would probably be my 50mm or 100mm primes, both are extremely sharp stopped down. The best that I know of is the new 300mm on a FF, check the ISO crops.
I know a lot of photographers that use 2x teleconverters, Juza is one example. I use one when I can't get close enough for a shot, it's better than cropping so I use it. I have the Minolta 600mm f/6.3 on a 60D so I know why bird photographers need 2x extenders for!
Edit.
I forgot to add, the bokeh does not change when using a teleconverter. You simply crop to acheive a longer focal length and narrower f/number, now if you get closer or used a longer lens with a faster aperture then you will be more background blur. In practice you will get less background in you image and usually a cleaner background which might appear to have more bokeh.
John.
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