The Rock River NM 2 stage makes a fine trigger once it is tuned. Edges cleaned up, polished. After that, they are as good as any of the more expensive ones. I have one in my Grendel and it is really nice, I was thinking about putting one of the Hi-Speeds in, but after I tuned it up and tried it out I decided to just stay with it, since all I wanted was a smooth, distinct, and light two stage trigger.
The best drop in has to be the Geissele. They make one for every need and purpose too. I have their 3gun in an M4 and it is amazingly fast. I'll probably use them again on the next build too. They cost more than the Rock River, but depending on the use, they may be worth it.
I have a Jard in my FAL. It is pain in the neck sometimes and I had to modify the radius on the back of the hammer and use a stronger spring on the disconnector. One I got it tuned up, it works pretty good, but the bolt still slightly snags the hammer. I'd suspect the AR trigger to be less fussy, but you still have the set screws to deal with. Only get an adjustable trigger with set screws if only plan on using it at ranges and competitions. For defense, you want a non-adjustable trigger. The RR or several of the Geisseles fit this bill (and are good for competition too).
Finally, if you have a good AR trigger, a factory one, you can fit and tune those to be superb 4.5lb triggers. I did one using just a fine Arkansas stone and a cotton polish drum on a Dremel. You have to use a hammer and trigger that do not have finished engagement surfaces, Oly is an example of what not to use, DPMS makes a cheap example of what to use. I've turned a DPMS trigger into a pretty nice single stage trigger.