Alright I fixed it.The brain isn't on the poll, which is my answer, him and the Bee.
The easy to beat part is great for all the people who like to speed run.My favorite pre-hardmode bosses are the Brain of Cthulu & the Queen Bee. They are fun bosses that require little items to beat!
For me, I think it has to be Skeletron Prime. I think that in the early days, that was the pinnacle of bosses, and you had to have your stuff together before taking him on. While I really like the Eye because it is the first boss, I put my vote for the first sort of 'final boss' in my mind.
Oh whoops. I meant to put Skeletron, but just added "Prime" at the end.PRE Hardmode bosses.