Updated in April, 2020
$300 is a price cutoff that a lot of people seem to have, so I watch this category fairly carefully and a lot of the scopes I see introduced in this price range are not particularly worthwhile.
It does not mean that they are all crap, but I see a fair number of questionable decisions made in terms of stuffing too many features into a budget design without getting the fundamentals worked out.
You can find a ton of scopes with all the bells and whistles for $300, but I would stay away from stuff that seems to be too good to be true.
For the basic precision/tactical applications I have yet to see anything for $300 that would appeal to me more than the various SWFA Classic fixed power scopes with MilQuad reticle; 6×42, 10×42, 12×42 and 16×42. The latter is a little more magnification than this optical system can easily pull off, but if you are looking for something that gives you a bit more magnification together with mechanical reliability, this is pretty much it.
For hunting, there are several perfectly decent options to look at with some new entrants that are very promising.
For a general purpose hunting scope, Vortex Diamondback 3.5-10×50 is a really decent options as are some Leupold VX-Freedom variants, like the 1.5-5×20 and 3-9×40. Historically, I have not been a fan of Leupold’s lower end scopes, but as prices go up, these stay under $300 and they are worth looking at in that price range.
Sig seems to be revamping they product line, but Whiskey3 scopes with an illuminated dot are really decent options. They have surprisingly well worked out illumination.
One new scope that I have not yet tested, but plan to is the new Burris Fullfield IV 3-12×56. Between a 56mm objective and illuminated reticle, it should be an excellent low light hunting scope on a budget based on a very quick look. It is here conditionally until I do a full test.
With red dot sights, there are now many really good options under $300 with Holosun 500 series probably being at the top of the list for carbines. If you have astigmatism, their gold dot sights are worth looking at. They used to OEM for a lot of people, but it looks like they are putting a stop to that. I think they still OEM for Sig and doign a couple of things for Primary Arms (if you use a magnifier, their Paralow with ACSS reticle is an interesting option). I have also had very good luck with HiLux MM-2.
For use on handguns, I really like Shield Sights, but most of them are a little more expensive. However, their new SMSc is under $300 and is perfect for the recent crop of narrow slide 9mm handguns for carry, liek Springfield HellCat. It would be at the top of my list.
If you are looking for a more conventional manual adjust micro red dot sight, Vortex Viper, Burris Fullfield III are perfectly decent. I have two Hawke Micro red dot sights that are likely made by the same OEM that have been absolutely bullet proof for a couple of years. I would definitely give them a close look.
Magnifiers: the new Vortex Micro 3x can be had for exactly $300 and it is hands down the best magnifier I have seen for this amount of doe. If you want to stay closer to $220, Athlon Midas 3x and 5x magnifiers are quite nice. That’s definitely the best 5x magnifier on a budget I have seen to date.
There are a few prismatic scopes under $300, but I have not been terribly impressed with most of them to be honest. Vortex Spitfire 1x is pretty decent and a good alternative to a red dot is you have astigmatism.
The one to beat among 1x prism scopes might be the new SwampFox Blade, but it is here conditionally, until I do a full test. It does look promising.