There are several criteria to answer this question: The continuity of borders throughout the ages, the feeling of nationhood, etc. It's therefore very difficult to have a accurate answer. One should never forget that when we throw ourselves into comparizions the risk of arbitrariety is bigger. I'd like to think it's my own country since from the begging and without any ambiguity independence was the goal. Moreover there allways have been a sence of indentity which it was not found on other similar experiences. Example It is wrong to indentify Scotland as the oldest nation since the clan divisions endured to the 18th century. FrankenReich is not France, whose modern simbolic shape began with Jean D'Arc and was latter completed By Louis XIV and by the Franch Revolution. The same applies to the notoriously divided "German Empire".
I dare say that one common feeling of portuguese people is that of "premência": The urgency to retain our own identity, which leads the poetic feeling of aspiring yet to something more perfect. In the words of Fernando Pessoa "Falta cumprir-se Portugal" Something like "Portugal is yet to be accomplished"
