also, could anybody recommend different shapes?
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Most look pretty good for low poly rims.
A few comments, just my opinions though.
The 1st row right rim just messes with my eyes. 
2nd row, 2nd rim seems very weird.
2nd row, 3rd rim has all 6 spokes at different angles. If you copy the top wide spoke and the bottom narrow spoke then rotate the copy 120 degrees around the center, then copy them again and rotate those another 120 degrees it will look much better.
Also, some rims seem to have a cylinder just inside the tire like a normal car rim, but others just seem to have the spokes attached to the tire itself, which looks a little strange.
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