You're on the G-guage forum so I think we'll tell you that G is best for outdoor use.
Rex
The Home of Articulated Ugliness
I am going to answer this two ways. If you mean the track gauge (width), 99% of garden railroaders in the states are using "G" gauge, 45 mm track (also known as #1 gauge).
If you mean track code (the height of the rail) I would use code 332 for several reasons - its the most commercially available, if you are running on the ground the added height allows your trains to go over obstacles better (twigs, acorns, gravel, etc.) and its the strongest, handling foot traffic the best. If you want a more true prototypic look, go with code 250 rail.
-Brian
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