The go-to red dot while keeping a budget - SIG SAUER ROMEO5 1x20mm 2 MOA Red Dot Sight

I have purchased a few of these for various builds and for the price, you can't beat it. Sure, there are some cheaper options, but it's not worth saving a few dollars to gamble on an inferior optic. Given, this isn't on-point with higher-tier red dots, such as AimPoint, but for the casual shooter, you won't notice much difference anyway.