The Land Raider, because I found its STCs and designed it...
Land Raider Crusader. Insane transport capacity, laughs in the face of plasma, has assault cannons, and is basically a bunker on tracks.
It's got to be the only true Astartes IFV for the win here.
Land Speeder Storm is a close second, Centaur third.
AUT TACE AUT LOQUERE MELIORA SILENTIO
My favorite is, and likely always will be, the trusty Chimera. The reason it's my favorite: Amphibious.
Also, I'm utterly shocked it wasn't on the list! The thread's title says "What's Your Favorite Imperial Dedicated Transport [emphasis added]," yet Space Marine vehicles dominate the list. There's more to the Imperium than just Marines.
There is one direction: FORWARD!
Definitely the Chimera as my go to battle taxi, my steel legion feild 5 of the armored buggers , with infantry inside , they make for a credible , MOBILE threat against almost anything yet arn't overcoat which makes room for more Russes , hellhounds and my lone medusa "with proper enclosed compartment"