If I answered this this question a few months ago, I would have said the Canon 400 f/2.8 IS, but searching through the POTN Buy/Sell forum a few weeks back, I found it for $2300 under the asking price so we bit the bullet and bought our dream lens.
Here is our reasoning. Much like Gordon and Bob's reasoning, I'd move up to the 400 over the 300mm f/2.8L. Heres why, with the 400mm you've got a great start, granted a heavy one (11lbs), but a great start for wildlife. This lens is also the crem-de-la-crem for the sports shooters which is who I bought ours from. He shot for Arkansas State Football, and the lens is what I call a 6 packer. Meaning after 6 beers she looks beautiful. Sober, it has quite a few scratches and scrapes on the white body as it came from a working professional. The glass is in pristine condition and all the motors and stabilizers work great.
So back to why this is my dream lens. If we add a 1.4x, we're out to 560 on the full frame 5D, and 896mm on the 30D with the crop factor. If we wanted to get even closer to the animals, then a 2x converter would put us at 800mm f/5.6 on a full frame or an astounding 1280 f/5.6 on a crop factor body and still have auto focus and a pretty fast lens.
By buying used and saving the $2300 off the new price, we can get both the 1.4 & the 2x extender along with a Wimberly head ($500!) to hold this monster stable and still be saming money in the long run. Oh, and I need to get a Kinesis Long Lens bag to carry this thing around. All of Canon's big lenses come with a aluminum suitcase, but that isnt practical to go out hiking with.
So you just got the same talk Cindy did and it made her say "Ok, lets get it!" What do you think.
Canon 400mm f/2.8L IS $6,800 (B&H Price)
Canon 500mm f/4.0L IS $5,600 (B&H Price)
Canon 600mm f/4.0L IS $7,220 (B&H Price)
Canon 800mm f/5.6L IS $10,799 (B&H Price)
Our lens, 400mm f/2.8L IS $4,500 Used. Wimberly Head $500 if I have to buy one brand new. I'm waiting to shop around and find one used like I find everything else. Kinesis Long Lens Bag $199, Canon 1.4 & 2x Extenders $280 each. Total spent to outfit with Dream Kit = $5,759. I know I'll find both the extenders at about half that price, as I see them all day long on POTN and Fred Miranda for $100 off retail price. Just waiting till we get back on the road before we make the purchase.
I'm still $1,041 off the Retail Price for the new 400mm and when we're ready to get to Yellowstone for the summer, we'll have the full dream kit ready for action.
If I was to just talk about dream lens which is unaffordable to anyone except the ultimate pro, it would be the Sigma 200-500 f/2.8 Electronic Power Zoom & Built-In LCD Panel for Focal Info....this puppy will set you back a cool $25,000!! but B&H has a $2,500 instant rebate incase anyone is on the fence for ordering.