For a long time, I’ve just put on DejaVu fonts and been done with it. Generally good enough Unicode coverage for me. But I know it’s been years since DejaVu’s been updated, and I wonder what’s very common today.
[As for the terminal, I’m guessing it’s usually still the standard fixed Unicode fonts?]
Taking a quick glance at the font packages I have installed, I find the Liberation family, Freefont, the old MS core fonts, a couple of Bitstream Vera Sans variations (including Deja Vu), and the ancient URW fonts, plus a couple of CJK-specific fonts, since I need those characters just often enough for their absence to be noticed.
Freefont has decent coverage of what was in Unicode as of ten years ago, and so in combination with the CJK specialty fonts covers most common writing systems worldwide. I’m not particularly concerned about things like Anatolian hieroglyphs, a couple of hundred less-common emoji, or the Bitcoin symbol being missing.