It's kind of fashionable to reflexively say Barry Smith, but there are a number of great stories in the later run by Thomas/Buscema. Particularly in the SAVAGE SWORD OF CONAN issues that were around 50 pages, where the extra length allowed the stories to really flow.
I'll list one of each that I love. From the CONAN THE BARBARIAN color comic, issues 19 and 20, "Hawks From The Sea", and "The Black Hound of Vengeance". That nicely explores issues of religion, of Conan's uncivilized nature that separates him from the others he fights alongside, in some ways more savage, in some ways more noble. And with intelligence as well as bloody battle scenes, the tragedy of war.
From Buscema, one that stands out for me is "The Haunting of Castle Crimson" in SAVAGE SWORD OF CONAN 12. A tale of the supernatural, revenge, and some wonderful twists, that ends with remarkable optimism for a Robert E. Howard Conan story. And Buscema's art is beautifully embellished by Alfredo Alcala, as Buscema's work is in most of the first 35 issues or so of SAVAGE SWORD. Buscema's work, particularly on SAVAGE SWORD, has a more pulpish feel to it.
I think both Smith and Buscema often suffered on their Conan stories from bad inkers. Most of Barry Smith's work is inked by Sal Buscema, who would not be my first choice over Smith. But as a new guy on his first series, he was probably not in a position to make demands in that department. It was very fortunate that Dan Adkins was inking the "Hawks From the Sea" issues, where Smith had reached peak form, with scripting to match.
Unfortunately on Buscema, he was most often inked by Ernie Chan. Others have raved about what a great combination they were, I fail to see it. And would always prefer another inker on Buscema. Preferably Alcala, Dezuniga or Nebres.
So who do you prefer on Conan? And on what stories?
Thanks to Dark Horse, all these issues are back in print, in collected trades.