It depends on your positions, whether you're standing or on the ground (and if you're on the ground are you in mount, does the other guy have guard, ect). In my experience the easiest and most versatile submissions on the ground are kimura/americana/keylock submissions that lock up your opponent's elbow and shoulder. Once you find the one you like (The americana is the easiest for me, since it's simple and I'm not a grappler) and drill it a few times, and from a few different positions, try it in a sparring setting. This family of submissions isolates your opponent's single arm against both of yours, which makes overcoming strength differences easier.

The armbar works on the same principle, except instead of two arms you lever your entire body against their one arm.

Honorable mention goes to the rear naked choke, which is the closest thing I've ever seen to a "go to sleep" button on the human body.