For a 45-degree offset sight you're going to want a smaller red dot like a Trijicon RMR, Vortex Razor, Leupold Delta Point, Burris Fastfire, etc. An Aimpoint or EoTech would work, but they'd be bulky, heavy, and awkward. Keep in mind that an offset sight is a back-up sight, so you're probably already running a scope as a primary and therefore you don't want your offset sight to be too big or heavy.
That brings me to my preference of 45-degree offset sights. Being that the whole idea of 45-degree offset sights is that they're used for back-up, I like iron sights for that role instead of something that's electronic and uses a battery. I like fixed 45-degree offset iron sights like the ones made by Dueck Defense:
45-degree flip-up iron sights like those made by Knights Armament are OK, but being flip-up means they're not quite as strong as good fixed ones and they take a moment to flip up when you need them.
45-degree offset sights are very practical if you're running a magnified scope as a primary optic. They allow you to make quick close-up shots that would be very difficult with the scope. Tilting the rifle is very easy and was totally natural for me the first time I tried it. And if the offset sights are the right height (like the Dueck Defense ones), they'll be the same height as your primary optic (if you mounted your primary optic at the right height) and that means you have the same check weld, making it much easier to transition to the offset sights.