Stihl. Chain Saws - Chainsaw Product Selector - STIHL USA
You can buy the others at any store which decides to sell them, but you can only buy Stihl from a Stihl dealer. The reason is service and maintenance.
Northern Tool & Equipment isn't going to fix your Husqy. Once you buy it, you're on your own. Fat Tommy's Chainsaw and Bicycle Repair Shop might be really good at patching up your Murray 21" mower, but Fat Tommy doesn't attend training or have factory reps show up at the shop every time something new comes down the line.
From an initial-quality standpoint, Stihl can't be beaten. For service and customer support, there is no other company with a similar reputation or program, for that matter.
I grew up felling and cutting trees, and have run everything from climbing saws up to a 2-man saw with a 5' bar (I held the handle at the tip of the bar.) If anyone ever owned the 1960's Homelite XL-12 with the 36" bar, you can appreciate what a big, heavy, powerful saw is.
I own either 4 or 5 Stihls at this moment, and would never buy another brand. I also have a grunch of other Stihl professional equipment, and the same goes for them.
Did I mention Stihl?