It's kind of hard to compare Rhinos and Razorbacks to Chimeras, since they're available to two different armies, and are used for very different things. As raw transports go, Rhinos have the upper hand, I think-- they're cheaper, and the contents are usually much more survivable than guardsmen anyway, so the lesser protection on the front facing doesn't seem to be that big a deal.
The Chimera, on the other hand, is armoured enough to deliver guardsmen into close range, if you really want them there. As Hesp said, though, Chimeras are better off used as anti-infantry platforms. Even without a heavy stubber, a Chimera is capable of spitting out six shots, which is a fair number for anti-infantry purposes.