For me - something I can use both digitally and printed - 300 dpi/ 12x12 in size.
I love collecting papers - I have over 1000 different papers (been collecting for years) everything from Christmas to shimmers and oils.
I use them to augment, sometimes to make a specific background for a card. Sometimes a specific shape to cut out or stripes.
Quality - I look for a designer that I know makes quality designs. I had the unfortunate experience buying from a designer off Etsy who made antique papers - but did it in a way the slice lines showed up when printing. I learned how to clean that up but still, it was quite an eye opening experience.
Also - be careful of folks who make tiles (web seamless tiles) into full paper sizes. It used to be a common occurrence back early 2000s, and they look horrific at 300 dpi printed.
DB has amazing papers - I can’t even keep up with my selections on downloading from the PLUS section alone.