The best argument (which might have been expressed here steady...I think) is 'because I just do.'
It's not as weak rhetorically as it sounds. It centres the belief on the person themselves which is better grounds for discussion than appeals to circular or drawn out philosophy.
The worst is trinitarian Christianity (see afleitch passim) which at it's core is half a step away from the same mythos as Zeus' paternal exploits (though to be fair, at least he made a physical appearance.)
I wouldn't consider any of the various Trinitarian doctrines an "argument for the existence of God". You're right that they don't work very well as one, but I think that's because that just isn't what they are.
I was referring to it being a 'gateway' into a Christian belief in God, particularly for those who have an 'accepting Jesus' (as...) heavy induction.
