Plus Widths (40mm-50mm) - These rims while not technically true "fat" sizes deserve honorable mention, especially for those of us trying to build super light wheels on the cheap or those of us who want to build a bike with loads of traction but will never need to take full advantage of the float fatbikes can offer. These are obviously great with "plus" or 3" wide tires but they can also work well with 4" wide fat tires. They round the profile of the tire a bit which cuts down on float but it makes the bike transition from upright to railing corners very quick and easy. Lots of riders like this option when trying to get a set of wheels in the weight range of a proper carbon fat rim but without spending a ton of cash. Just don't try to mount anything wider than a 4" tire on them.
65mm Widths - This is by far my favorite size to build for year round fat biking. They are light, they make leaning the bike into corners super easy, and still give good enough float for MOST riders. They are great everywhere but super deep sand/snow. They can even take 5" tires without issue.
80mm Widths - If you mostly use your fatbike in sand/snow but still would like it to handle reasonably well on fast techy singletrack, this is a great rim width for you. They have most of the float of a 100mm rim but without hurting the way your bike feels on tight twisty trails too much. They can take all tire sizes and take you nearly anywhere you would like to go. This truly falls into the jack of all trades, master of none (except maybe groomed snow) category. If you don't know what size to order after reading this, get 80's.
100mm Widths - Right now this is about as big as fat rims come, these really turn your bike into a monster truck. You will roll over/through nearly anything in your path, they give more float in deep sand and snow than anything else out there. They also flatten your tire out pretty significantly meaning more of the knobs are in contact with the ground to give you as much traction as possible. The downside to all that is they make getting the bike leaned over and into a tight corner take a bit more effort and increase rolling resistance on firm surfaces but this is OK because this rim is to take you to places a bike can not normally go. That's not to say you can't ride technical singletrack on them, because you absolutely can and some people love the way they feel there, just know it will take a bit more effort to move the bike around underneath you. Naturally being the biggest, they are also the heaviest so unless you are looking at rims like the HED BFD, expect to pay a bit of a weight penalty.
Obviously personal preference can make all points above moot but for most riders, this will give you a really good starting point to selecting the best rim width for your bike. Always be sure to take into consideration frame limitations, if a 5" tire barely clears your frame on 80mm rims, don't expect to move up to 100mm wide rims because the wider your rims are, the wider your tires will pack out. Feel free to email me via the contact form with any more specific questions.