I'd say you have to narrow this down somewhat to get a helpful reply. For example, what sort of prior building/modelling/flying experience do you have? What sort of skills would you like to learn? If you like the WWII birds you might want to pick a subject that has the usual features, such as rivets, panel lines, etc. and pick a model that has these in an easy way. I chose the Eindecker partly because it is boxy and therefore a pretty straight-forward build for someone like me. Also I wanted to learn how to work with SolarTex (vs. monokote), how to simulate aluminum cowls and side panel, how to do a pull-pull cable system, stitching, rigging and all the other things typical of WWI planes. I don't know how good my Eindecker will look (or fly!) when I'm done but I'll have learned a ton about scale building and that'll mean a better second scale effort.

Best of luck with whatever project you end up choosing.