As stated, Oak Trail is a walking only 9 hole course. It isn't an executive course, but a real course at a par 36. There are many who go around twice to make it 18 holes. Depending on the time of year for your trip, I wouldn't advise walking 18 in Disney.
We have played all of the courses extensively, including the now gone Eagles Pines and Osprey Ridge courses at Bonnet Creek near Fort Wilderness. In terms of difficulty, Palm is tops followed by Magnolia. They have the best pro-shop on property as well as the top practice green and two driving ranges.
Alas, my favorite meanders right through Old Key West - Lake Buena Vista. I would highly recommend this course if you're staying at OKW. It's a short (and free of charge) taxi ride to SSR and the Pro-Shop. Then they'll play literally through Old Key West Resort. There's an island par-3 and plenty of challenge to be found. Sure, it's not at the same level of difficulty as the Palm and Magnolia, but it's still VERY well-maintained. The greens are hard (read: fast) and the course has more than its share of challenges. Plus, you get to see some cool sites, as the course is really spread out and you pass by OKW, SSR, POFQ, POR, THV and Dis Springs. There are even several holes along the river with ferries passing by enroute to Dis Springs or the resorts.
I used to take golf seriously - too seriously. And back then I was a huge advocate for the courses over by the Poly. Nowadays I'll still play them, but the ambiance and vibe at LBV are much more my style.
Oh, and Chip & Dale's deli at LBV outside of the pro-shop has the cheapest food and beer on all of WDW property!