I surprised myself somewhat in voting for Dunn. It is really a close call for me among 4 players.
I have been convinced by others that Bender offers a unique skill-set and, despite his limited playing time this year and his young age, he has a relatively high floor as a good defender who can pass and make open 3s at the 4 and maybe the 5 some. Even the most optimistic options don't peg him as a first or even second scoring option on a good team, though, so I think this limits his ceiling.
I really wanted the pick to be Jaylen Brown. That is, I really want Jaylen Brown to be a Jimmy Butler/Paul George prototype, but I'm really concerned he never gets close. I am particularly turned off by how bad he looks under the advanced metrics and that he was unable to transcend (at least a little more) the less than ideal circumstances at Cal. This is a tough call because he is young, has an NBA-wing body, and was such a highly regarded high school recruit, which traditionally is a decent barometer of future success.
For me, that leaves Murray and Dunn. On first blush, Murray is the guy, because he can shoot, something the Celtics (and every other team) need. He also is young, was a top high school recruit, and was a standout at the Nike Hoop Summit two straight years. But given Murray's defensive limitations, I have trouble seeing him being able to share the court with IT. Those two are just a terrible combination defensively, and for a variety of reasons, I don't see IT being moved by the Celtics. I think it is much more likely they move Bradley or Smart (more likely Bradley).
By contrast, I can easily see Dunn co-existing with IT. Dunn is long enough to play defense on the larger guard, and he can allow IT to play off the ball a little bit, which seemed to work for the Celtics in the playoffs. I think Dunn's ceiling is probably the all-star version of Kyle Lowry. Murray's ceiling is probably is the all-star version of CJ McCollum, with a little more range. Those ceilings seem comparable to me, so I take Dunn who, somewhat counter-intuitively, is a better fit, and who is probably more ready to step in and contribute due to his age.
Maybe that is short-sighted. And Murray is probably a better fit next to Smart. But I think it is going to be hard to keep IT off the floor for most of the minutes the next two years, and I hope that whoever the Celtics draft at 3 will also be able to be on the floor with him.