BEST SPEAKERS FOR WALL UNIT

Obviously, you don't want a speaker that has a rear port.

Two speakers whose manufacturers' web sites show them on shelves are the Thiel SCS3 and the Wilson Duette. I haven't heard either of them, sorry, but both manufacturers have good reputations (if slightly controversial in Wilson's case). I would urge you to consider auditioning them yourself. Good luck.

North Acoustics Spirits. They do have a rear port, but specially built for close to wall and in a book shelf. There is a pair for sale right now on Audiogon.

Sonus Faber Concerto Home is front ported and may work. I use them close to a rear wall with good results. I think they are better if not too close to side walls, though. The model is discontinued as a monitor, but there seem to be some available here and there.

Vienna Acoustics Hayden Grand is also a front port design. I haven't listened to VA, so can't compare to the SF or comment on it more generally.