Husqvarna 61, 257
Stihl MS 250, MS290.
I have a friend that has a fair sized business (four crews), and I repair his equipment.
He uses the 250s for the ground crews along with his other pro saws, because they are inexpensive and they can do a lot of work. On my recommendation, he uses Stihl Ultra oil, and has never had a top or bottom end failure! This is a guy that makes a living with saws, so he knows a thing or two. A small investment compared to a pro saw, which he knows will eventually be destroyed by an employee. Normally they are retired after they are run over, etc, and then used for parts.
Just my 2 cents.