My first bit of advice would be to subscribe to touringplans -- but that's because I love planning, re-planning, over-planning and planning my planning. If the people you're advising are not like me (that is, if they're not the kind of people who keep lists of their lists), disregard.
I wouldn't even bother with "which parks on which days." Showing up early and tackling things in an intelligent order will trump any small rise or fall in crowd levels, and for the most part, they're all busier on weekends and holidays and they're all less busy on weekdays. They should just pick the parks they want, in the order they want. (*Do, however, check the calendar in case there are special events -- e.g., a RunDisney marathon or an early closing for a seasonal party or a private event -- that should be planned around!)
Best time of year: whenever you can go. If you can avoid all school vacations, summer break, and hurricane season, so much the better. When we could pull the kids out of school, early October and March/April (avoiding anything near Easter) were great times to visit. Now that we have to work around school schedules, late August (after the Florida schools resumed) turned out to be an incredible time to go, crowd-wise, although we had to resign ourselves to very hot and humid weather. The "yourfirstvisit" website has a calendar that gives great insights into the best weeks to travel in particular calendar years: I'd recommend taking a look at it.
For planning purposes, on a first or second visit, and assuming 1 park per day, my order of operations would be: Magic Kingdom first (stay late for the fireworks), followed by Epcot (opens late, so you can sleep in after the late MK night), followed by Animal Kingdom, followed by Hollywood Studios. Then circle back and end with Magic Kingdom, or hop there after a partial day at AK or HS. If there are only 4 days to work with and non-hopper tickets, then I'd start with HS, then Epcot, then AK, then MK. Whatever the order, I try to tuck Epcot in the middle because it's the most boring (YMMV), and to finish (and ideally also start) with MK because it's everybody's favorite and has the greatest number of things to do.