Daniel Barenboim has been my go-to for Beethoven sonatas since my undergrad years. I want to say it was one of the first complete sonatas collection on CD. I was able to pick it up cheap a long time ago too.
I was profoundly underwhelmed by Barenboim's Reith lectures. His talks came across as rambling and incoherent. Filled with the kind of "spiritual" hogwash I usually associate with charlatanism. Not to mention a few factual errors. I think sometimes conductors spend so much time waving the baton around and charming ignorant, culture starved patrons they begin to believe their own bullshit. That is what led me to seek out the Brendel.
After a couple weeks of poking around I found the Brendel collection for around $30 and snapped it up. A trusted friend recommended Brendel for all things classical and German many years ago and I remember really digging his Haydn sonatas.
So, wow! What a difference. Rhythmically and motivically clearer than the Barneboim by miles. I am hearing Beethoven's ideas for the first time seemingly. Brendel seems to understand every passage whereas Barenboim just kind of virtosos his way through more subtle parts.