Holosun, Vortex and Primary Arms for some of the BEST budget friendly RDS. I would avoid Truglo as the quality is a bit lacking. All good red dot is out of stock on Buds so go to the Primary Arms website and check out their offerings. Also, do you prefer "tube style" reddot or more open "window style"?. For tube style: SIG Sauer ROMEO5, Holosun HE403B, and Bushnell TRS-25 (less than $100). For open stye: Holosun HE509, SIG Sauer ROMEO3, or EOTECH. I hope this helps

This is always one of the hardest question to answer, because we don't know, "what it the rifle being used for?" Home deffense, Plinking? *what kind of range if plinking? Are you shooting 3 gun matches? What is your budget like? You ate looking at a cost of anywhere where from$30 to $10,000
If moneys no object id go with a Vortex Razor HD Gen3 1-10× this will run you about 2 grand, and is is a high end versatile LPVO ( Low power variable optic) or under a grand id get a Vortex pst gen viper, or around 300 id go with a Vortex strike eagle gen2 1-6 or 1-8.
If its a dedicated home deffense weapon id go with a reddot or holographic site like a holsun 510C, or of you got money to spend aim point T1, trijicon mro. Or a holographic site is good for close quarters as well eotech is the high end then holosun has a holographic, or Vortex huey is good too.
You should check out CDoes YouTube channel, in my opinion he has the best most informative reviews for optics on the web.
youtube.com/watch?v=Fc-iVN8Yc1I

Joseph, bar none, hands down, the Primary Arms with ACSS : primaryarms.com/primary-arms-2-5x-compact-ar15-scope-with-patented-cqb-acss-reticle-pac2-5x