Almost anything full art, specifically from masters sets. I have an unpopular opinion, that the Magic frames (both old and new) are the worst part of card aesthetic and a very popular opinion, that MtG art is top notch. If I can have more art and less frame I’m happy.

In terms of alt art treatments I think my favorite is the Zendikar showcase. I particularly like Roil Elemental, Admonition Angel, Avenger of Zendikar, and Canopy Baloth, but really almost all of them look fantastic.

The Strixhaven Mystical Archives arts are also really nice.

Edit: Oh I need to add to my answer. The Secret Lair art of Azami, Lady of Scrolls is fantastic. The other arts from that lair were decent but I really like this one.