I think Starlin's most obscure and least-known work is the stuff he did between 1977 and 1981 for DC, during the period he had a conflict with editor in chief Jim Shooter and left Marvel.

Here's an oversized wraparound cover Starlin did for SUPERBOY/LEGION 238,April 1978. A reprint issue, of a 2-issue story by Shooter and Swan/Klein from ADVENTURE COMICS issues 359 and 360, Aug and Sept 1967 :
https://i.pinimg.com/originals/8b/93/6b/8b936b07c844e3104b53227cbc0482f4.jpg
https://viewcomiconline.com/superboy-1949-issue-238/

Starlin also did covers and/or interior art on SUPERBOY/LEGION 239 and 250-251. Oddly credited as "Steve Apollo" in the last two issues.
https://viewcomiconline.com/superboy-1949-issue-239/
https://viewcomiconline.com/superboy-1949-issue-250/

At DC, Starlin mostly did interior art and covers for DC COMICS PRESENTS in 1980-1981. But he also did covers for a huge swath of DC titles from 1977-1981, including BATMAN, DETECTIVE, BATMAN FAMILY, DC SPECIAL SERIES, JLA, KAMANDI, HOUSE OF MYSTERY, HOUSE OF SECRETS, GHOSTS, SECRETS OF HAUNTED HOUSE, WEIRD WAR TALES and others.

[Linked Image from milehighcomics.com]
DC SPECIAL SERIES 12, June 1979

Why I've always liked DC more than Marvel, it allowed Starlin to do work in a huge range of genres, whereas his Marvel work was mostly limited to superheroes, some having a science fiction edge. But at DC, in addition to seeing Starlin's version of all the DC characters, he worked on science fiction, war, westerns, horror, and humor. So while not well known, Starlin's work at DC in this period gives a large selection of pages that are pretty far outside what he is normally known to work on, and at the peak of his talent.

In this same period, Starlin did some painted magazine covers for MARVEL PREVIEW (issues 13 and 14) , SAVAGE SWORD OF CONAN 26, and other black and white magazines.

Starlin's "Metamorphosis Odyssey" series in EPIC ILLUSTRATED 1-9.
https://viewcomiconline.com/epic-illustrated-issue-1/

THE DEATH OF CAPTAIN MARVEL (Marvel graphic novel 1, Dec 1981),
https://viewcomiconline.com/marvel-graphic-novel-issue-1-the-death-of-captain-marvel/

THE PRICE graphic novel for Eclipse (Oct 1981),
https://viewcomiconline.com/the-price-full/

and DREADSTAR (Marvel graphic novel 3, Oct 1982),
https://viewcomiconline.com/marvel-graphic-novel-issue-03-dreadstar/

after which he worked exclusively for Marvel/Epic a few years, doing the DREADSTAR comic series, issues 1-32.
https://viewcomiconline.com/category/dreadstar/

For my money, Starlin's greatest work. The earlier painted art issues in particular.