For me, there are two difficulties with Descartes' ultimate truth "I think therefore I am". Leaving aside any translation issues because of the fact that the phrase was originally conceived in French, then later slightly reformulated when he wrote in Latin, my first difficulty is with the "I" component and my second is with the "thinking".
The "I" problem occurs because of Buddhist philosophy which suggests that the self is an illusion and that we all belong to one conscious whole. Whilst I have not personally undertaken the years of training in meditation that seems to be recommended to reach such a sublime revelation, nor had a Zen-like instantaneous insight to the truthfulness of the position, "I" doesn't need to be an immovable object. Why not just "something"? This would sidestep the issue of the self as an illusion, and depersonalise the content of my own truth.
The "thinking" problem can be put like this. Am I thinking that I exist, or do I just perceive it? In any case, to me, it is not the thinking which is the important element (from my own understanding, this may have been more so to Descartes), it is the existence.
So my own first certainty is: something exists.
Because I cannot conceive of a way that this could be false, I shall call it my ultimate truth.
Straight away a hurdle presents itself. Solipsism. Am I alone in my thinking or perception that something exists? Am I the only conscious being that exists, with everything else just going on in my mind? It is a desperately lonely position, awesome in its potential, and utterly frustrating in its execution. Essentially solipsists find themselves in the role of a diminished God. All seeing and all knowing, but with powers which are severely limited. Solipsism is a desperate position to take, and there may not be a rational way to escape it. It requires a leap of faith to escape the solitude. My first assumption is therefore to assume I am not alone. Not because it must be, but because it must be for my own sanity.
My first assumption is: I am not alone.
Because I need to assume this in order to lead a half-way decent life, I shall call it my leap of faith.
It is possible that I could enter into constructive dialogue about my certainty and my assumption, but I predict they will both be positions which it will be hard to drag me away from. I am sure, however, that a far more interesting dialogue could be achieved around the general form of my findings. I have one ultimate truth and one leap of faith. Is it possible that this could be a general formula which would hold for everyone? We might find that we vehemently disagree about the specifics of our truths and our assumptions, but do we all have exactly one of each?
To summarise my own position ...
Ultimate truth: something exists
Leap of faith: I am not alone
... and in explicit terms, my theory is this:
in order to live well, we each recognise one ultimate truth and one leap of faith.