I'd target one of the guards - Lillard, Beal, McCollum.
Sabonis is a player who could be interesting also.
Not that interested in forwards. We have two already in Jaylen and Tatum. I want ball-handlers, shooters, passers. More guard skills.
John Wall is a has-been. Not a star. D'Angelo Russell is a high usage role player. Not a star. D'Aaron Fox is a maybe star but I do not think he is available for the type of package Boston can offer nor do I consider him a good fit for this team (dodgy shooter). Kevin Love, like Wall, is a has-been. Not a star. Porzingis is a solid player but is struggling to put his game together and I would not bet the house on him so little-to-no interest there. Westbrook is moving into that has-been pile as well.
Zion is awesome but would cost more than the C's can offer while keeping Tatum & Jaylen. KAT is currently in the same boat but that may change over next 18 months - still, it's a long shot that nobody offers better than a bunch of mediocre first rounders for him given how talented he is so I am not really considering him.
That leaves the forwards Siakam and Ben Simmons. Not sold on Siakam as the right fit - we need shooters, ball-handlers, passers; not another defensive SF/PF. Ben Simmons would be good but the C's trade package doesn't match up with Philly's desires so it would require a 3rd team to have any chance of nabbing him. I like the idea of Simmons as our interior big & main playmaker. That would be a highly dynamic and dangerous team. But like I said, difficult to make a trade package.
For those reasons, I feel the guards are the best targets. The two guys in Portland look the most likely to be available for what Boston can offer (all our future 1st round picks). Washington is going well so Beal is off the table for now.