I've found version 2.0 to be the easiest version to target and deploy since a lot of people have it installed already, If a sizable portion of your client base use Vista you might consider upgrading to 3.0.
Versions above that nearly always require an install which can be a pain for some users.
Edit: The "framework version going out of support" argument holds no water since 3.0 is an extension of 2.0 and 3.5 is an extension of that. By definition 2.0 will be supported as long as 3.5 is. Version 1.1 is the only version that is a completely separate runtime and is no longer supported.
