It's difficult for me to pin down what my most valuable book was. I had a very nice silver age collection as a kid, most of which mysteriously went missing not long after I "outgrew" funny books in 1972. The weird thing is that I didn't even notice they were gone for 20 years until I started collecting again in the early 90s. When I stopped collecting I stopped caring about comics and moved on to other interests. My mom never threw my stuff out so I don't know what to think. The only books that survived were my Detective Comics collection, including Batgirl's first appearance in #359.
I didn't have any of the early Marvel first appearances but some of the books I know I had are Avengers #2 and a beat up copy of #4 along with almost every other issue up to #106. I had the key FFs from 45-53 and a lot of lower numbers. I had the Canadian version of Spidey Annual #1 and most of the issues between 20 and 81. I had close to a hundred issues of Journey Into Mystery/Thor up to #206. I bought all of the premier issues in the 1968 Marvel explosion off the racks; Iron Man, Subby, Hulk, Cap, Captain Marvel, a complete run of Surfer 1-18, etc. I also had a bunch of mid to late 60s DCs, including Poison Ivy's first appearance in Bats 181, but there aren't many other big money DCs from those years.