I think Guardians covers Thanos well enough.
First Avenger is higher on the secondary list due to Bucky, The Red Skull, and The Tesseract. But Winter Soldier shows up in Civil War, the Tesseract is merely a macguffin in Infinity War, and knowing who the Red Skull is isn't essential to...
This is my list of essential MCU movies you need to see to follow along Infinity War:
Guardians of the Galaxy
Avengers: Age of Ultron
Captain America: Civil War
Doctor Strange
Thor: Ragnarok
These flesh out more of the emotional arcs and fill in more details about the Infinity Stones and...
Bautista IS a comedy savant. That line was ad libbed!
https://www.yahoo.com/entertainment/captain-america-iron-man-scene-wasnt-6-things-didnt-know-avengers-infinity-war-165510777.html
That makes three Infinity Gauntlets -- one in Odin's Trophy Room in Thor (the one Hela dubbed as a fake in Ragnarok), the one Thanos is wearing at the end of Age of Ultron, and now the one forged in Nidavellir around the same time Asgard was destroyed.
I though the most sublimely silly exchange in the entire movie was
"WHERE is Gamora?"
"I'll do you one better, WHO is Gamora?"
"I'll do you one better, WHY is Gamora?
Drax's line is loopy and amusing enough, but the delivery just makes it a classic. Bautista is a comedy savant.