DISCLAIMER: MY OPINION ONLY
There are many good brands out there as suggested, but you need to decide what you want it for.
Whether you want it for Home defense or professional use or just casual range plinking or hunting may make a difference in the price paid vs quality relationship. For professional or personal defense use I always recommend Colt LE carbines, these are somewhat expensive, but you get consistent quality control and Proofing. They are not as expensive as Les Baer and Wilson Arms for example. I would not include these two in my list because while they are excellent rifles they are overpriced IMO.
My personal preference, in order I would purchase would be as below, regardless of the type of rifle you want. However please understand that I place priority on quality control and proofing over price everytime. This is not the same priority that everyone has.
1. Colt
2. Stag/CMT/RRA (many components by same mfg - CMT)
4. Bushmaster
5. Armalite
6. DSA
7. Fulton Armory
8. DPMS
There are many other good mfgs out there like CMMG now making their own stuff, Sabre, etc. Always a good idea to post here first to purchase to see what experiences members have had. Just be prepared for varying opinions.
The Only one I would advise to avoid at all costs are Hesse Arms and Vulcan products, reports of poor quality control are numerous on these, and I have seen some of the issues first hand.
Also be careful of used guns of questionable origin, remember that just because the receiver may say Bushmaster or Colt, everything else on the rifle could be a cheap kit or other parts of unoown origin and dubious quality. Never assume it's all original unless you can prove it with markings or it's a new rifle in the box.
ETA you asked about Oly. I have an Oly, 2002 mfg, has been a good rifle but replaced the barrel with a chrome line 4150 steel barrel. Quality control/materials is not as good as ABC. OK rifles.