I voted for reliability, but there is no way to know if an individual rifle is reliable until it proves itself, so I would go with a BRAND known to be of consistent high QUALITY over time. These three are tied together.
If I buy a complete AR15 anytime soon, it will probably be a Colt LE model, because that brand is known to be reliable among folks I know, plus my pencil-barrel AR15A2 Govt Carbine was flawlessly reliable over time. I sold it because I got to where I couldn't focus on the too-close front sight any more, and wanted a different version of AR to help remedy that. Originally, I wanted something with a longer sight radius, but when my command staff (PD) decided to let us use optical sights, my goal has changed to a model that facilitates optics, and may or may not be a carbine-length weapon.
OTOH, I am not a Colt snob, and may go with another brand.
FWIW, I did build up a "parts" AR with a Colt HBAR upper receiver, S&W M&P15 lower, an RRA LRPK, JP Enterprises bolt assembly, and other high-end components. If this iron-sighted target-shootin'-oriented AR runs reliably, I may decide to purchase another, more fighting-oriented upper for it, rather than a whole AR weapon. I will still go with a brand with a reputation for quality and reliability.