Z - smallest scale, highly expensive, and not too much available.
N - second most popular scale, lots of stuff available. A “serious” scale, but it’s still easier to model the post-1960 era than anything else. A good scale if you like modern trains, but not very good if you’re into the massiveness of trains, or if you like steam.
TT - almost a dead scale, it’s between N and HO. The only modern items available are German prototypes. If you’re into being different from the crowd, this is the scale for you!
HO - the most popular scale, it has the largest selection of models. It’s also a good scratchbuilding scale. If you’re really into steam, there’s no other serious choice.
OO - slightly larger than HO, it’s the only scale to model in if you’re into Briti***rains. There’s not that much available though.
S - There are TWO S scales: scale and American Flyer (tinplate). AF is the most popular and the best supported, and Sn3 seems to be the second most popular. S is a niche scale, but there’s more support for it all the time. A good scale if you like narrow-gauge trains or want massive trains that don’t take up as much space as O.
O - As with S, there are two O scales: scale and “lionel” (tinplate). While it may be toylike, three-rail has a LOT of support, so items are easy to find. O is the best comercial scale to model in if you’re really into mass.
G/F/Q…“Garden Scale” - This is gigantic stuff, but most of the models are of tiny prototypes. G is mostly used as a fantasy garden railroad, but I’ve seen some nice “serious” indoors layouts done in G. While there’s a lot of items available, it’ll still require a lot of compromises and scratchbuilding, and really is nore suitable for outdoor layouts.