Phalanx may always be my go-to whenever the topic of good/bad video game box art is brought up—the cover is a meme by now—but quite frankly, I like it; it's telling a story and it's unique.
Other than that, most of my favorite box art are from games from the '80s–'90s where they used to be mostly drawn/painted and often burrowed from movie posters. Probably the best era in regards to that trend.
chef-kiss.gif /mamamia
Katsuya Terada.
Tom duBois. Artist for Konami/Ultra Games. Here's a really good video on him: https://www.youtube.com/watch?v=dw-SPY4n0s8
So many other greats from or around that era: Bob Wakelin, Mick McGinty, Frank Cirocco, Marc William Ericksen, Roger Dean, Tim White, Cliff Spohn, Steve Hendrickson, John Harris, Don Ivan Punchatz, Boris Vallejo and Julie Bell, Yoichi Kotabe, Yoshitaka Amano, Akira Nishimura, Hideaki Kodama, Bengus, Shinkiro, Akiman, Daigo Ikeno, Kinu Nishimura, Haruki Suetsugu, Akira Toriyama, Naoyuki Kato, Yuji Kaida and so on.