I consider Descartes' cogito to be one of the most ingenious discoveries and undeniable truths in philosophy. And yet it's not considered that way by most, if one may judge from their lack of enthusiasm. If I recall, Thomas Nagel went as far as to claim in a paper that it's a circular argument and so it proves nothing, much less that I exist.

It's worth noting that just because I can doubt something being me, doesn't mean it's not me. It might mean it's not necessarily me (I could have different bodies), but it doesn't mean it's not contingently me (kill this body and you kill me too).

Overall, anything beyond the cogito itself falls into the area of consciousness, and it's a really hard topic. Could robots have minds? Apes probably have minds but do cockroaches have them? How do I know for sure another person is not a robot? Am I just using a kind of 'argument to the best explanation' when I infer that other humans, too, are conscious? Quite tricky topics...