I dont believe there is a best shovel at all, but rather the best shovel for the application, whatever that is. Any machine will sit under the yarder and load- they did it with line machines (Bucyrus, P&H, Manitowoc, Link-Belt, etc), then old hydraulics- Prentice D600's and Husky XL-300's, etc. Slow but capable nonetheless.
For flat-out shovel-logging the Thunderbirds and Madills couldn't be beat. The TB 942's, 1242's, and Madill 3800C's just flat-out smoked eveything in thier classes with huge raw horsepower and giant pump-flow. The downfall is they burn insane fuel and need lots of attention mechanically due to the horsepower and flow. Between breakdowns however, you will get more logs than everybody else is with thier Link-Belts and Hitachis. Can you afford those logs? That's up to you.
The playing field today I think is basically pretty even- Cat, Komatsu, Deere/Hitachi, Link-Belt are all decent machines that will give you good service for 10,000 hours.
There are still the price-point machines out there too- Kobelco, Doosan, Volvo, etc and from what I've seen they work fine but you wait longer for parts and you get a little less resale for them. I dont know if it was really fair to put Kobelco in this category anymore- they've come a long way and the Link-Belt/Kobelco are very close in terms of price and durability. I dont think Volvo is ther yet with durability- the machines I've been around were terrible. Lots of technology (too much?) and not enough iron.
Here's the real truth however: The Feds have thier boot on the throat of industry in this country and right now your log loader manufacturers aren't worried about you getting any logs. They're worried about how the heck they will meet all the ridiculous emissions standards coming down the pike. You're an innocent bystander who is about to start suffering the consequences of this nonsense. Machinery prices are going through the sky and machine reliability is about to go down.
I'll close with this: The best log loader didn't have to worry about meeting any emissions standards!
And John C is spot-on: A lousy/careless operator will destroy the best log loader, whatever you decide that machine is.