I'll tackle this from the assumption you're looking for the best lens, money no object (as you indicate).

The best lens will be the Canon 500L f4 IS, 600L f4 IS or 800L f5.6 IS mounted on a gimbal mount on a very solid tripod rated in the 20+lbs area. Which of these is the one for you comes down to a couple things.

Focusing speed: The 800mm will have slightly lower focusing performance than the other two since its maximum aperture is f5.6 and less light will be hitting the sensor, especially an issue with a 60D (as opposed to a 1D series body).

Field of View: The 800mm will be incredibly tight as far as FoV is concerned especially at 1.6x crop factor, it'll be great for capturing small birds far & really far. The 500 and 600 will be a better bet to capture wildlife near & far.

Weight: The lenses weigh a fair amount 600mm (11.5lbs), 800mm (9.5lbs) and 500mm (8.5lbs). If you plan on hiking at all with this lens consider that in the equation.

Overall: I've found that most people shooting wildlife in general wind up getting the 500mm because it gets you awfully close, covers small & larger subjects, and is above all the lightest so you can hike into position with it. Its more versatile. However, I've found that people who are interested solely in birds favor the 600mm as it gets you that much closer to (much smaller) birds and deal with the added weight by just not moving around as much.

Having used and owned the 500 and 600 myself I found the 600 to just be way to effing huge, and the 500mm was the sweetspot but at the time I was shooting wildlife in general, not birds in particular. I haven't shot w/the 800mm personally but looking at the specs and FoV I think I'd still pick the 500mm. Though it should be noted I was shooting at 1x and 1.3x crop factors.

I'll tackle this from the assumption you're looking for the best lens, money no object (as you indicate).

The best lens will be the Canon 500L f4 IS, 600L f4 IS or 800L f5.6 IS mounted on a gimbal mount on a very solid tripod rated in the 20+lbs area. Which of these is the one for you comes down to a couple things.

Focusing speed: The 800mm will have slightly lower focusing performance than the other two since its maximum aperture is f5.6 and less light will be hitting the sensor, especially an issue with a 60D (as opposed to a 1D series body).

Field of View: The 800mm will be incredibly tight as far as FoV is concerned especially at 1.6x crop factor, it'll be great for capturing small birds far & really far. The 500 and 600 will be a better bet to capture wildlife near & far.

Weight: The lenses weigh a fair amount 600mm (11.5lbs), 800mm (9.5lbs) and 500mm (8.5lbs). If you plan on hiking at all with this lens consider that in the equation.

Overall: I've found that most people shooting wildlife in general wind up getting the 500mm because it gets you awfully close, covers small & larger subjects, and is above all the lightest so you can hike into position with it. Its more versatile. However, I've found that people who are interested solely in birds favor the 600mm as it gets you that much closer to (much smaller) birds and deal with the added weight by just not moving around as much.

Having used and owned the 500 and 600 myself I found the 600 to just be way to effing huge, and the 500mm was the sweetspot but at the time I was shooting wildlife in general, not birds in particular. I haven't shot w/the 800mm personally but looking at the specs and FoV I think I'd still pick the 500mm. Though it should be noted I was shooting at 1x and 1.3x crop factors.

Side note: Avoid zooms if you want the best, zooms are not as sharp and focus much slower, the 100-400 in particular. If you want something less expensive consider the 400L f5.6 or 300L f4 IS (the 400L 5.6 being sharper and focusing faster).