I'm curious about what other people chose as their favorite sourcebooks of all time...
I had picked the Encyclopedia Magica Tomes.
I picked the Encyclopedia Magica (so what if it's technically it's 4 books

)
So many magic items with so much story....
I loved it how all the items were finally collected into one source book(s).
And using the random table generator, add in quirks, etc, and this blows away any DMG treasure table!
It also helps that my players and I have used those DMG tables so often, we had them practically memorized. Add in a luckstone's 10% on rolls, and it was boring.
Now using the Encyclopedia Magica, that same 10% on a 1,000% table, and so many items, attributes (and quirks), and the players felt like they were getting a "NEW" item each and every time.
And their utter disappointment when they get a regular +3 longsword.