For commercial fonts, many sites make you beg for information, even though one would expect salesmen to exhibit the opposite behavior. MyFonts supplies more information than most, allowing you to do an advanced search for fonts with Greek language support tagged “polytonic.” Both criteria are necessary, because searching for Greek in the “language support” field returns the many fonts supporting only modern Greek. Unfortunately, the tags reflect the discernment or lack thereof in the site’s users, which means that many polytonic fonts are not listed among the results.
For example, the search returns Figgins Sans and Pragmatica, but not Hypatia, which is an interesting sans serif with polytonic Greek and everything needed for proper transliteration of Semitic languages (very few fonts support that). P22 Underground Pro is another that should be listed but isn’t; it supports an extraordinary range of languages and comes in many weights but has no italic.
Be sure to examine the table of glyphs for the fonts listed when you search that site, because I have not found searches perfectly accurate. That’s true also for another kind of search you can perform on MyFonts: choose the field “available characters” in the advanced search form, and type or paste some uncommon character into the search box.
Among free fonts, Ralph Hancock’s Hyle is very attractive; it has no ligatures and you need to write a feature file to use its ligatures, but that’s easy in
luatex. Aroania, like the Greek Font Society’s Neohellenic, is a revival of Scholderer’s New Hellenic, but based on Verdana. Source Sans Pro also supports polytonic Greek, as long as you get it in TeX Live or directly from Adobe, not as optimized for the web by Google. Also in TeX Live are DejaVu Sans, Fira Sans, Carlito, FreeSans, and Biolinum. The Ubuntu fonts and Lato (not the Lato in TeX Live, but the full version from the designer’s site) have broad support for languages. And the Noto fonts aim to support all scripts encoded by Unicode.
You may have other sans serif fonts that support Greek already on your system. At least on GNU/Linux, you can find them from the command line by typing
fc-list :lang=el. You will then have to inspect them to see whether they support polytonic or only monotonic Greek.