I just moved to the Appleton area a few months ago and have had a chance now to play most of the courses in the area, many of them several times. Ranked in order of my favorites:
Plamann : A gorgeous 27 hole course in great shape with a variety of challenges. A must play if you're in the area.
O'Hauser : My 'home' course. Each 9 plays like a different course, one is fairly open, the other mostly wooded. A little flat, but a tough course (for me, anyhow), I just can't seem to play all 18 well, it's one 9 or the other from day to day. Keeps it interesting. Thursday League plays here.
Winnebago Cty Park in Oshkosh : Brand new, only 15 holes, and completely flat, but wind almost always is a factor. Make good use of water and trees where possible. Since it's new, it's still in great shape and seldom busy.
Gringon : A nice course, but over-rated in my opinion. Maybe I'm just bitter because the park administrator yanked the pin for #4 due to vandalism (both the vandalism and the park's reaction to it rub me the wrong way). Lots of elevation changes which is nice. Makes for a good workout.
Telulah : A lot of variety, some narrow wooded fairways, some wide open, nice elevation changes. Beautiful views of the Fox River in places. Not rated higher in my list due to erosion and vandalism. Wed night league p[lays here.
Red Arrow in Oshkosh : Poorly marked, highly eroded and vandalized. Nice layout though. I suspect it was once a very nice course.
There are a bunch of little 9-holers around too, but i've only played two of them, Kimberly High School and Pierce Park. The Kimberly course has some pretty awful tee pads, sunken mini-ponds with ankle-breakers for borders and not too well marked, but a fun, short course. Pierce Park was fun too, well marked, challenging in parts and one I'll likely play again.