I have been through just about everything I could find before around 2010 and a few of his newer ones and in my opinion I would just read everything.
Of course his supposed Opus are the dark tower books (which is what originally drew me in) and there are a fair few other books based around that world and gives nods to each other, some bigger than others.
Ones that I enjoyed a lot personally though (dark tower aside) are Talisman, Black House, Desperation, Needful Things, Cell, Under the Dome and definitely 11/22/63.
My least favourites were probably Cujo and Misery but people seem to rate them so that is more personal I think.