Okie dokie. You asked for it. ;-)
Sam is very much like me. I, too, was considered the "smart" younger sibling, the one who was expected to do so much (my father once suggested I go to the Naval Academy in Annapolis, Maryland and become the first female Navy Admiral *eye roll*), who became a disappointment when I dropped out of college, got married, and had a kid. I was the one who rebelled against my parents plans for me.
So, yeah, when I started watching SPN, Sam bored me. I knew that story. I knew what choices he would make, how he would lash out at John, blah blah blah. Dean was more a mystery to me. Plus, he made me laugh. That's always a good thing in my book. :-)
Years later, Dean is still my favorite, but Cas is getting closer to taking that spot. Dean doesn't make me laugh as much as he used to. Plus, he's straying awfully close to "alpha male moron" in recent seasons. IMO, of course.
ETA re: flaws
While Dean is my favorite character, I don't pretend his flaws don't exist. Nor do I excuse them. Nor do I get angry when the writers use those flaws to forward the story. It's all part of storytelling and having flawed heroes as the main characters. I do not believe that Sam or Dean are "Mary Sue"s. I think the story would suffer immensely if they were.
For context, I "blame" Dean for taking the Mark of Cain. I don't "blame" the writers because Dean is flawed. I need Dean to be flawed for him to be a compelling character. The characters have to make mistakes for decent storytelling. I would get so bored if Dean did the right thing or made the right decision every time; never got knocked down in a fight; or never got outwitted by a bad guy. Where's the dramatic tension in a story like that? Yes, I know that Dean will "win" in the end, but how does he get there? That's the story I want to see.
Honestly, I don't get the concept of "blaming" the writers when Dean makes a mistake. I get blaming the writers when the writing is bad, but Dean being human, with human flaws, is not bad writing, IMO.
I don't think either character is "propped". To be propped, one must not be able to stand on one's own. I would never say that about Sam or Dean's characters.