Ahhh... considering you can count the major publications on less than one hand...it really depends on what era and gauge you intend to model
Garden Railways is the most accessable and perhaps the best place for a newbie to start. Magazine covers narrow gauge and standard gauge, old and modern, diesel and steam, live steam and electric.
Garden Rail is very good but thinner than GR, very similar to GR but its exclusivly British/European based, heavy emphisis in past on LGB but does cover other brands, modern layouts as well as older steam based layouts
Steam in the Garden is great IF your doing live steam, if your interested in big modern railroading, is not much use.
Narrow Gauge and Shortline Gazette is a finescale dedicated magazine with heavy emphisis on narrow gauge, again great for what it covers but again if your into big modern railroading, its not much use.
Narrow Gauge Down Under, a Aussie based magazine very similar to the Gazette, but thinner, dedicated to narrow gauge in the land of Kiwis and Walkabouts.
Timber Times, niche magazine dedicated to modeling logging lines, similar to the gazette.
As for me I get 2 magazine subscriptions, GR and the Gazette. I do buy the occasional TT, NGDU, and Garden Rail when I can find them