Good question socalgardengal
Hard to say as the closest thing i like to collect next to seeds are the seed catalogs themselves!
In terms of selection and the quality of the catalog itself, the following are the ones I'm constantly looking over:
Tomato Growers Supply Company
Seed Savers Exchange
High Mowing Organic Seeds
Baker Creek Heirloom Seeds
Southern Exposure Seed Exchange
Seeds of Italy
For rare/fragrant flower seeds the "Select Seeds" catalog.
And though not for seeds, the full-color 120 page Rose Handbook/catalog put out by David Austin is a must!
For Trees, the "Tree-Book" catalog by the Arbor Day Foundation is really informative, and for you Japanese Maple lovers, the "Essence of the Tree" Japanese Maple catalog, it's amazing the variety of Japanese Maples!
All these catalogs are free btw.
Ok I think that's enough catalogs from me, I can suggest more...but i'll just wait to see what the other tomatovillians mention first.