The first Warlock epic, from Strange Tales through the solo Warlock series. The finale, shunted off to an Avengers and then a Marvel-Two-In-One (I think?) Annual wasn't quite up to the level of the rest of the story, IMO, although they are amongst the better Avengers comics ever done. But they were Avengers, rather than Warlock stories, and I never liked Rubenstein's inks over Starlin.
Starlin's Captain Marvel would be a close second, including but by no means limited to The Death of. To be honest, I don't think as highly of The Death of as most readers do. Still a good piece of work, but nowhere near as compelling as the earlier series.