I've never really endorsed the ontological argument, in any of its forms. One version, however, that has interested many contemporary philosophers is Alvin Plantinga's modal ontological argument. More novel still is Robert Maydole's version of the argument, which appears in The Blackwell Companion to Natural Theology. I have to say that, while I'm not persuaded that these arguments are rationally compelling, there very well may be something to Plantinga's claim that the modal ontological argument at least provides rationally acceptable reason to believe in God. Plantinga himself puts the argument this way:
1. There exists a possible world in which maximal greatness is instantiated.
2. A being is maximally great if and only if it is maximally excellent in every possible world.
3. A being is maximally excellent if and only if it is omnipotent, omniscient, and perfectly good.
4. Hence, a maximally great being exists in every possible world.
5. Therefore, an omnipotent, omniscient, and perfectly good being exists.
Perhaps surprisingly, premises (2)-(4) are relatively uncontroversial. Essentially, the argument merely defines maximal greatness in terms of maximal excellence, which in turn is defined as exemplifying the properties of omnipotence, omniscience, and perfect goodness. It seems quite natural to say that there cannot logically be a greater being than this.
The proof also makes use of the so-called "S5 axiom," which states that if something is necessary in one possible world, then it exists in all possible worlds (necessity = instantiation in every possible world). This axiom is likewise uncontroversial.
The premise that is the most contested is (1). The first time I began researching this argument, I was caught off guard that anyone would question the bare possibility of God's existence. After all, wouldn't the opponent of the first premise have to demonstrate some contradiction with the idea of a maximally great being in order to soundly reject (1)? I believe this is the case, and while even a great many atheistic philosophers reject the notion that there is some inherent contradiction between the divine properties, that doesn't stop others from attempting to prove that there is such a contradiction.
This is why, in God, Freedom, and Evil, Plantinga first deals with J.L. Mackie's "logical problem of evil" before delving into the modal ontological argument. He systematically demonstrates that there is no contradiction between the reality of evil and the existence of a maximally great being. This gives us prima facie reason to accept the first premise of the modal ontological argument. It seems that in order to produce a defeater for (1), the atheist must be able to positively undermine the possibility of God's existence. This is quite a task.
On the other hand, Maydole has apparently tried to demonstrate that (1) is not only rationally acceptable, but that we can know its truth. I have yet to give his version of the argument more than a cursory look, so at the moment I'm unable to comment any further.