The clear #1 in my lifetime is Hagler. I've seen the other guys on film and understand these lists are entirely subjective. Any of the Greb, Monzon, Robinson led lists are fine with me. Really you can't go wrong arranging those four names in any order.
I have Ward just behind Calzaghe on my all-time middleweight list, and narrowly ahead of Frankie Liles :think
I want to say Greb, but cant say for certain without film, Hagler and Monzon are definitely with him though. Those are top 3. Number 1 is interchangeable.
Rankings are by their nature subjective, and how a fighter looks can affect how you view them. For example, judging Ketchel by newspaper reports, resume and how he was regarded by observers at the time, he would rank with the greatest middleweights of all time. The footage of him though is underwhelming, and even accounting for the quality of the film, he doesn't appear to be the force of nature that legend holds him out to be. Greb can still be assessed by his (amazing) record, along with the footage of great fighters he beat like Walker and Tunney. However, without film of his fights we have less information on which to form an opinion.
I'm sure there are a few here who are still scratching their heads if they haven't already hit up Google...