I'm not going to knock Colt, as Colt makes a fine AR, but one thing to keep in mind when buying a Colt M4 is some accessories/parts for it have to be specifically made for a Colt M4. (They're not hard to find though.) As far as quality goes, Colt is really good though and tend to be nearly perfect in regards to their quality control.
I learned how to shoot AR's with my father's 1970s A1 Colt and it's still functioning great over 30 years later.
My AR is a rifle length Bushmaster I upgraded with a Bravo Company bolt carrier group. It's been flawless for years.
I'm not a carbine fan. I prefer the 20" barrel rifles. I learned to shoot a rifle length AR and could never get used to the shorter length between the front & rear sights on the carbines. They have a nice handy and maneuverable aspect to them though so I can understand why a lot of folks really like 'em.
If you can't find a Colt, there are some other good makers out there such as Lewis Machine & Tool, Bravo Company, LaRue, Sabre Defense, and others who produce some great higher-end upper receiver groups which you can add to your lower receiver of choice and have a great AR.
Or there's other ones, less expensive but still good, like Bushmaster, Stag, and RRA. My brother and I have had no problems of any kind with our Bushies.
I tend to go for the old classic ABC (Armalite, Bushmaster, Colt) lineup. Heck I like pretty much all AR's.
I'm going to build my next one. I want a 16" mid length and I want to go with a LMT upper with foliage green MagPul furniture if possible.
AR's are great. They're like a model kit which makes liberals cry.