Lamb Lies Down on Broadway. A top-5 pick for me (of any music). Saw Genesis do the entire album live (plus encores of course) at the NY Academy of Music (on Broadway!) back in Dec. 1974. A life-changing experience, they were that good. I think it helped that I was in the 11th row (I kept notes of all the shows that I attended).
Next in order: Selling England by the Pound, Trespass (highly underrated IMO), Trick of the Tail, Foxtrot, Nursery Cryme, all good. Can’t get into any post-Gabriel stuff past Trick of the Tail.
I agree with the commentary re: Hackett. Also a highly underrated guitarist that brought so much to Genesis. IMO after he left, the band was done.