I recently asked “Who is the Justice Leagues greatest foe? I don't mean the DCU but rather the Justice League. Anyway let me know your opinion” and I great options. Now I would like to do a poll and here from who do you think is the Justice Leagues main nemesis from that list.  

Here are the Listing that were picked below:   

1. Darkseid
2. Lex Luthor
3. Doctor Light
4. Injustice League (Specially Major Disaster....or Bruce.....well he could be )  
5. Starro
6. Amazo
7. Crime Syndicate

These are my choices that I found mostly appearing in anything Justice League

1. Despero
2. Felix Faust
3. Joker
4. Solomon Grundy
5. Royal Flush Gang
6. Icicle
7. Floronic Man
8. Jean Loring
9. Overmaster
10. Professor Ivo
11. T.O. Morrow
12. Shade
13. Sonar
14. The Key

This is the link from my previous forum:
 http://www.comicvine.com/justice-league-of-america/65-31815/who-is-the-justice-leagues/92-570087/  

hm.. that's a tricky one. the bad guys change a lot for each version of the justice league. Darkseid, Luthor and Joker come to mind simply because they are such heavy hitters in the DCU. But I think if you're looking for someone specific to the JLA, I'd have to pick Despero, followed by the Crime Syndicate.