I'd consider OpenGL 3.3 today's lowest common denominator, if you are after backwards compatibility. It removed the intermediate mode pipeline and introduced the Core profile, making it a good candidate for the lowest thing to target.
In OpenGL 2.1 you can pull in essential features like shader4 and UBOs via extensions, but 3.3 made it core. New ways to interact with OpenGL like DSA are generally available via extensions as well. It's "not recommended" mainly because Apple is the odd one out in removing support for it.
