This is a subject that can get as complicated and insider as any Fantasy Sports League draft. Trust me, I know from experience (glorious, wonderful but nevertheless exhausting experience).
The long and short of it is that just like any topic regarding postmodernist list-making, there are a few well-tread tropes to expect:
1. Pioneering first-comers that set the template for every iteration that follows.
2. Political entries that earned status by the tenor of the times and have been grandfathered in despite aging merit.
3. Truly brilliant works that embody the textbook foundations of a medium.
4. Truly brilliant works that violated every tenet of the foundation and reinvented it.
5. Miscellaneous entries that are often vilified/exalted for the same properties and will never be universally one or the other.
6. Newer entries that haven't been sanctified by establishment culture and so keep jockeying for position in the canon of "classic status"
Fortunately, most of this has already been done for the casual observer:
https://www.wga.org/writers-room/101-best-lists/101-greatest-screenplays/list
https://www.indiewire.com/2018/04/best-screenplays-scripts-american-1201952200/
http://www.filmsite.org/101greatestscreenplays.html
http://whatculture.com/film/10-essential-screenplays-every-aspiring-screenwriter-must-read
https://www.scriptreaderpro.com/best-screenplays-to-read/
Here's a pamphlet of names who continually recur on the lists of best writers in the medium--the Hemingways and the Steinbecks and the Stephen Kings if you will:
Dalton Trumbo
William Goldman
Paddy Chayefsky
Charlie Kaufman
David Webb Peoples
Steve Zaillian
David Koepp
Paul Schrader
Terry Southern
James L. Brooks
David Mamet
Robert Towne
Herman Mankiewicz
Ernest Lehman
Quentin Tarantino
Lawrence Kasdan
The Epstein Brothers
The Coen Brothers
Christopher McQuarrie
Kevin Williamson
[EDIT: Aaron Sorkin]
[EDIT: Damien Lindelof and David S. Goyer are popular and frequently-working screenwriters but their work is often crap]
From here on in, it's an endless grab-bag of possibilities, largely dependent on individual taste. Lots of the recommendations I would make would be a bit dense and craft-heavy that aren't generally accessible to a lot of people because when you're a film geek, you marvel at the technical skill and not so much the overall enjoyability:
Get The Gringo is an enjoyable film, but the script is not all that good;
Glengarry Glen Ross is a masterpiece of screenwriting, but some people might find a bunch of guys talking and arguing and cussing for 2 hours boring.