There should be little doubt that Mike Reid is the single greatest DT (he even gets my vote as the greatest overall player) of all time. Heck, as a first-year (sophomore) MLB he set a record that still stands: 3 safeties in a game vs Maryland.
Contrary to one poster's comment, Steve Smear was not selected to any 1st team All America list in 1969. A former End, Smear was an ideal compliment to Reid at DT. However, as a tandem, these 2 would rank, IMO, a notch below the nearly unblockable duo of Bruce Clark and Matt Millen.
"Salt and Pepper," as Clark & Millen were called, moved from LB to DT in 1977 as sophomores and began a reign as dominating forces that SMU's coach called "the best DT duo I've evere seen ... nobody can block them both at the same time." [roughly paraphrased quote ... I can't recall the precise phrasing].
But one pair that many Penn State fans seem to forget is the Randy Crowder-Mike Hartenstine duo in 1973. Crowder did make All America, and Hartenstine was named NBC's Defensive POY in '74. Hartenstine was so quick off the snap, that he often tackled Stanford's QB Mike Boryla before Boryla could even begin his backpeddle.
I don't think it is fair or even relevant to rate a college player's career by looking at his NFL career. Some players like Dennis Onkotz never receive the credit they deserve simply because they never excelled in the NFL. I think Clark and Hartenstine, for example, sometimes lose recognition because they both played DE in the pros, and Millen played MLB. But as college DTs, Clark & MIllen were just fantastic!