I enjoy much of Roy Thomas' work too.
But oddly, to my knowledge, Kirby never collaborated with Roy Thomas.
What I love most from Roy Thomas is his CONAN, SAVAGE SWORD OF CONAN, and SAVAGE TALES work.
And also his later CONAN THE ADVENTURER with artist Rafael Kayanan.
And "Bran Mak Morn: Worms of the Earth" adaptation with Tim Conrad, in SAVAGE SWORD 16 and 17. (reprinted a few years ago as a graphic novel)
And "Almuric", in EPIC ILLUSTRATED 2-5, again illustrated by Tim Conrad, in 1980-1981 (also reprinted in a collected volume, by Dark Horse)
I talked a lot about Thomas' contribution, and showed covers for many of these stories in another topic:
I also enjoyed ARAK by Roy Thomas in the early 1980's, a deliberately Conan-esque creation by Thomas, when he left Marvel and went to DC in 1981.
And as you say, his ALL-STAR SQUADRON, INFINITY INC., and YOUNG ALL-STARS runs for DC in the 1980's, which resurrected the JSA to its modern prominence.
If I were to recommend just one issue of Thomas' ALL-STAR SQUADRON, it would be
ALL STAR SQUADRON ANNUAL # 1 (1982)
Which links the origins of three Golden-Age characters, Wildcat, The Atom, and the Guardian.
It shows Thomas' vast knowledge of the characters he writes, on any series, and the way he can expand on that knowledge to explore ways to bind them together and tweak them for interesting little unexplored twists.
Although Kirby never collaborated with Thomas, to my knowledge, Kirby
did produce a rather un-flattering portrayal of Thomas in MISTER MIRACLE # 6 (1972)
Where Kirby created Funky Flashman, a fast-talking con-artist prone to Stan Lee -esque alliteration and backslapping. And Flashman's groveling ass-kissing lapdog assistant
HouseRoy, which are widely recognized as Kirby's expressed opinion on the subject of Stan Lee and Roy Thomas.
