PSE, PF and APS pretty much in that order, except for Columbians you're only getting a cert to verify no hanky panky done to make condition better than it is. If you're going for MNH that's important, if just unused a lot less so. Prices have been real stable for at least the last 10-20 years, so not a good investment when compared to more traditional like stocks or bonds. I'd expect that to continue for the foreseeable future since the hobby is shrinking slowly. PSE does grading as well as certs and the highest graded examples do have a strong premium. It tough to find an ungraded stamps that will grade higher than 80 or so without it already having a PSE grade, just due to the premium on the higher grades.
I think if I were thinking of buying as an investment rather than to build my collection I'd go for something that's more of a rarity than those, maybe a really well centered set of the 1869 pictorials, the 1860 90 cent, #1 & #2 or some of the postmaster provisionals. You missed out on the recent Kelleher auctions that had Bill Lang's collection, though there may be some more to come.
