My most successful [as in, I actually won one game in 7th with IG, instead of loosing horribly] was using Vets. Key to them being able to take maximum damage output per transport.
And Carapace armour. The Emperor loves a man in Carapace armour more than any of his other subjects. I can't say enough good about Carapace. Cover saves are disappearing in my world. Which isn't so bad for MEQ, but when Bolters, Gauss, Pulse and Eldar non-Scats are punching through your armour save, things have got to change.
In Pods, while supported by Dreads in Pods. With Scouts in camo. And a Techmarine to boost that cover save. I mean, not
everything ignores cover.
And technically I was cheating, because I was twin-linking any Emperor's Wrath shot that was within range of a Vox, instead of just one unit. And I won with 2 infantry models and one Dreadnought on the board. Well, and 3 Drop Pods that were out of range to do anything. My opponent had his WK about 6 inches from the objective [Thanks for ending on turn 6!] and some JB that had failed to get a decent "jump move" in their assault phase.
For the difference in the base unit cost, the additional BS and weapon selection is too good. And then you get Carapace. Literally, the only time I use Platoons is to get cheap bodies to bubble-wrap tanks [access to Conscripts] to give them a mobile cover save. Ablative armour, I guess you could say. Inability to move effectively makes them a liability. Without Carapace, there's nowhere for me to move, that my FAST moving opponents can't immediately capitalize on.
So yeah, a Chimera is wasted on a Platoon squad, really, so I say Vets for any and all situations that aren't specifically just to give a cover save to something more valuable.