How many Fantastic Four covers did Jim Steranko produce in the 1970s? Only three to my knowledge.
Fantastic Four #130 was produced during Steranko's flurry of 1972 covers for Marvel. This issue featured the return of the Frightful Four, with an interesting twist. Medusa, once a member of the Frightful Four in 1960s, had switched sides and become a member of the FF. Taking Medusa's place? Thundra, the Femizon who wanted to make babies with Benjamin J. Grimm. The Thing looks great on this cover.
Fantastic Four #131 featured the answer to a mystery that began in Avengers #104--whatever happened to Quicksilver? The last we had seen of the silver speedster, he had heroically sacrificed himself to defeat a Sentinel. Seriously injured, Pietro saw something horrible approaching him. Then he vanished, leaving the Avengers and his sister Wanda to wonder where he had disappeared.
This issue gave us the answer! Quicksilver had been approached by Lockjaw, the teleporting giant bulldog from the Inhumans. Lockjaw brought Pietro to Attilan, where he was nursed back to health by Crystal. Crystal melted Quicksilver's icy exterior and he fell in love. Naturally, when Johnny Storm discovered them together, a massive fight ensued.
It was a pretty darn good soap opera moment in the Marvel Universe. Unfortunately, it had the side effect of removing Quicksilver from the Marvel playground for a number of years.
While the 1972 FF covers are good, they don't seem like iconic Steranko. But the cover to the Fantastic Four Marvel Comics Index in 1977 is really groovy and psychedelic, man. Nuff said.