Horford isn't on the level of Gasol/Duncan, but Garnett is better than West and roughly equivalent to Nowitzki
I'd rank...
Gasol/Diggler
Horford/Garnett
Duncan/West
The Garnett & Duncan combos are so close...really flip a coin
I don't know if I'd rank Gasol / Dirk first. They're the worst rebounding of the three listed combos, and the worst (combined) defensively. Are their offensive contributions enough to trump? I'm not sure.
For me, none of these trios particularly stand out. Each one of them are formidable in their own way, and your preference is entirely going to lean towards what you're looking from in your bigs, as well as what sort of players they're surrounded by. What is certain is the fact that all three of them are definitely among the most talented frontcourts in the league.
That being said, I think it's very shortsighted to say Dirk/Marc are the worst combo because of their rebounding and defense. Big men aren't the only ones who rebound on a team, and rebounding isn't a commanding factor to begin with. The most obvious example would be Boston in recent years (Bass & KG), but Miami and San Antonio are two of the most successful teams in recent years that don't crack the top half of the league in rebounding.
Furthermore, Marc's DPOY might be a fluke, but so was his absence from the all-NBA first defensive team. The perception of Dirk being a "bad" defensive player is simply not true anymore because he has committed to that end of the floor a lot more at this stage in his career, and the numbers back it up; furthermore, Dirk has demonstrated that his diminishing defensive deficiencies can further be masked playing alongside an elite defensive center. Again, I'm not going to say any of these three combos are superior because each are dominant in their own way, but to say Dirk/Marc is the worst defensive combination of the three is absolutely false.