On the whole, I'm a Marvel guy when it comes to Superheroes. I love Batman, but Superman, Flash, Wonder Woman, Martian Manhunter, etc these are characters that fall flat for me. I love Geoff Johns on Green Lantern, but the movie was so terrible that I don't expect them to be able to save it. Marvel did such a good job with the individual movies fleshing out each character that the Avengers became an even better movie for it. There ARE good JLA stories (Kingdom Come, Tower of Babel), but these exist on such a more fantastical scale than the Marvel universe that it becomes tough to transition to the screen.
This is the Wikipedia list of JLA enemies. How many would be villains that a casual fan would know?
Amazo
Amos Fortune
Anti-Justice League
The Appellaxians
Asmodel
Brainiac
Crime Syndicate of America
Copperhead
Darkseid
The Demons Three
Despero
Doctor Destiny
Doctor Light
Doomsday
Eclipso
Epoch the Lord of Time
The Extremists
Felix Faust
Funky Flashman
Gamemnae
The General
Gentleman Ghost
The Injustice Gang
The Injustice League
Joker
Kanjar Ro
The Key
Kobra
Lex Luthor
Libra
Magog
Maxwell Lord
Morgaine le Fey
Neron
Professor Ivo
Prometheus
Queen Bee
The Queen of Fables
Rama Khan
Red King
The Royal Flush Gang
The Secret Society of Super Villains
Shaggy Man
Starbreaker
Starro
T. O. Morrow
The Ultra-Humanite
Vandal Savage
The Wizard
The White Martians
I think going with Luthor is a cop out, as is the Joker. Doomsday has absolutely no emotion and would be a worse version of Bane from Batman and Robin. Braniac, as written in the Geoff Johns run, is interesting but is traditionally a Superman villain. This leaves us Darkseid. Who I find interesting. But I don't trust the studio to give me any of the good bits about Apokolips while also giving me 5+ characters of background. Plus, I think most people who saw Avengers will just think it's THANOS