Any ringnecks, Red or yellow goldens, lady Amhearst are the easiest to keep. Silvers and Elliots take a little more effort but are about the same keeping wise . GBWF forum and Softbills for sale are the best in my opinion.
Totally agree with the above. You can't go wrong starting out with Goldens - http://www.gbwf.org/pheasants/golden.html They are affordable and easy to find. You will get a feeling on pheasant behavior and then decide if want to move up to the more challenging species. Silvers are good beginner birds, but some males can be aggressive. I wouldn't recommend Elliot's as a first bird, try Reeves first. I'd recommend Amherst as well, but their purity is just so messed up right now, do some homework before you & who you purchase your stock from.
As always, the larger the better!! Most aviaries are rectangle or square in shape, if rectangle - a min of 8 x 16 and square, 12 x 12. I've seen them kept in much smaller, but the more room you can provide, the better and you'll have a nice tail on the male. I also like taller aviaries, where one can easily walk into and you can add layers of branches for the birds. It is important for any pheasant aviary to add plantings, rocks, logs, & enrichment (I think I type that a lot!!) - anything to provide a naturalistic habitat for the species you want to keep will make for happier, healthier birds. Not to mention, a well planted and landscaped aviary is also much more appealing to eye than a bare floor. It can be done rather cheaply as well by using native plants and forest material, no need to break the bank. I would use willows (which would need constant trimming), transplanted cedar & pines from the woods, rocks & logs found also found in the woods.