Which canon body? FF or APS-C?
The Panasonic 100-300mm has the field of view of 600mm lens on a full frame body, though above about 250mm the IQ starts to drop. And because of that for me it would be a toss-up between the 70-300 and the 45-200.
Olympus has a 40-150mm f2.8 which is getting extremely good reviews and with the matched 1.4 teleconverter would (I am sure) beat the 100-300 for ultimate quality even when cropped to the same FOV. But it won't have image stabilisation.
Olympus are also releasing later this year a 300mm f4 and that sounds very tasty (well... except for the potential price, that is).
If you are comparing to APS-C body then 600mm on APS-C is equivalent to about 450mm on the GH4 and there is no such lens.