Of course, I could ask who conjectured the most, but conjectures can later be shown to be either true or false, so perhaps who has the most unresolved conjectures, in publication? More than that though, also considered would be mathematicians with a reputation towards conjecture for discussion.
Another related question would be the person with the most disproven conjectures, in publication? I'm also looking for interesting accounts of someone being shown to be wrong in conjecture while alive, yet still go on to write mathematics, hopefully of high quality, despite the setback.