BJ,
Personally I have a warm spot for Luther and Lutheranism due to being baptized Lutheran.
Roman Catholicism had a authoritarian side that Luther was right to criticize. It has a certain overbearing dogmatism like its idea of Papal Infallibility. Even the idea of the Infallibility of the Magisterium is excessive.
But on the other hand, Lutheranism and the Protestantism that followed in his wake (I am including Calvinists and Baptists in this) also has a certain dogmatism. I doubt that Lutheranism considers Luther's Five Solas "infallible", but the Protestantism that followed Luther seems to treat them as rather dogmatic, or maybe it's better to call it "axiomatic." And the Five Solas are even questionable from a non-Catholic standpoint, since the Anglicans, Methodists, and EOs don't openly teach them.
Which of the Five Solas do you consider the most Foundational? It seems to be "Sola Scriptura," because Luther emphasized the Bible as the source of all doctrines, which would include the other Solas. "Sola Scriptura" has been said to mean that the Bible is the only "final", "infallible" source of authority, but it's not really clear that this was Luther's definition of it. I don't know if Luther would agree that the Bible is the only final or infallible source, as opposed to God revealing something, like the details of the Last Judgment, after the Bible was written. And for that matter I'm not sure that the RCs would even disagree that the Bible is the only final, infallible source. One RC (maybe on this CARM forum) told me that if Sola Scriptura just meant that the Bible was ranked at the top of written Church authority, then he could agree with it. For instance, a Catholic could consider the 4 Gospels and a Papal "Ex Cathedra" statement both "infallible", but the 4 Gospels would rank higher in authority.
Luther came from a school of thought in Western Europe called at the time "Humanism" that tried to interpret documents as self-explanatory texts. Another scholar who belonged to this method was Erasmus, a Catholic academician. Luther explained Sola Scriptura by saying that he wanted to follow the meaning of the Bible in order to determine doctrines and to judge whether other Christian leaders were right. When it came to using the Church fathers to understand what the Bible said in the first place, Luther seemed to go in opposite directions. At one point he counseled not using them, but other times he said to use them, and in practice he used them himself to understand the Bible.
The real weakness in the Sola Scriptura idea is that if you just take the Bible Alone, then the Bible's meaning is frequently unclear in reality. I can't even say that the Bible authors meant for their writings to be self-evident, because when they wrote texts like Daniel's Book and John's Apocalypse, the writers encoded or encrypted their meanings in symbolism. Luther did not openly say AFAIK that the Bible's meaning is obvious on all important topics, but the fact that it is not obvious shows a weakness in his fundamental "Sola Scriptura" principle, ie. in order to establish doctrine, in reality the Church can't just pick up the Bible and go by the Bible alone as its only text. This is at least in part because you actually need other texts to understand the Bible in the first place.