The "Ham Sandwich which is greater than any that can be conceived of" argument you are so fond of has been rebutted by more than a few philosophers throughout history, so although you are fond of it, it is still a weak premise to base a belief in God on.
And never really satisfactorily rebutted, despite your constant claims to the contrary.
According to you. When you have an emotional attachment to the conclusion of the argument, you're going to be less willing to relinquish it: you should agree with that, whether or not you think the argument holds.
The ontological class of arguments can fall for several reasons. There's one set that rely on 'greatness' to be a measure of something objectively that transcends and predates even God: absurd. Then there's the like of Anselm's which tries to go from "the idea of God exists in the mind," (as ideas are all that can exist there) to "the actual God exists in reality," with no explanation: the best you could say is that 'the idea of God exists' which no one denies. Then there's a whole host which are only relevant theoretically: about as meaningful as assuming there's a biggest number, and showing that it's one (which can be done quite simply). Start with faulty assumptions, you get a wrong answer.
And not one person has ever shown a trait of the 'greatest possible...' is something as basic as sentience (and I'd argue that it's impossible to show that), even if you grant it has to exist. That renders the point moot, in any case.
But, at least we've gotten you down from atheism.
Also, outside of what is essentially technical jargon that noone speaking outside of a classroom is concerned with, atheism and agnosticism are not mutually exclusive. This has been pointed out several times. You can't force your definition on others when your definition is only meant to be used in a classroom and similarly technical settings.