There are quite a few circles which have done their share for the RoyAi doujinshi community. Some of them have done one or two books, while other are really prolific. Here are some picks for you.

Hakai Noise [website] is absolutely gorgeous. Not as old as the others, but releasing new doujinshi in every big event. It's got some of my favorite art, plus serious stories, color illustrations... they've got it all, really. Highly recommended.

Star Cluster Drops [website] is another soft spot of mine. Probably not everyone's cup of tea, but for me it's got amazing sketchy art plus some of the best characterization for both Roy & Hawkeye. Excellent.

527 [website] is cute beyond words. Cartoonish wonderful constant art, mixed with usually high-spirited stories, sometimes bordering the gag and gen. Their Roy is delightfully expressive and other characters' appearances (Roy-tachi, Ed, Al) are always fun. This circle makes me extremely happy.

INSIDExNOISE [website] is probably one of the oldest and most popular RoyAi circles. I find its edgy art beautiful, to which you have to add great improvement throughout time. Their stories are usually serious angsty ones with various ratings (some smut included). Although not as profilic as it once was, there are still copybons released from time to time.