I haven't used the Meganium Crest personally (or even bought it, or obtained a chikorita... ), but I don't see why it wouldn't be useful. Leftovers healing combined with a damage reduction effect that extends to allies in Doubles (presumably 25%, since based off of Friend Guard), as well as the hidden buff of having a Held Item that can't be knocked off? Seems like both a bump in its durability and a way to more reliably facilitate its clerical or SD sets.
I have used the Oricorio, Relicanth, Zangoose, and Cofagrigus Crests myself so far. Of those, I can say:
Lastly, I want to give a quick shoutout to the Claydol Crest. This thing changes its Sp. Attack stat to be the same as its Defense (thus reducing how thin you need to spread your EVs), and increases the power of Beam Attacks by a 1.2x multiplier. More importantly, though, it also completely negates the charge-up tun for Solar Beam regardless of the Weather, and the recharge round of Hyper Beam. That's right, it can spam Hyper Beam with impunity! The only other time you can off something like that off is if you're fighting in the Glitch Field, and that has the caveat of needing to actually kill the foe! This seems like a best-of-both worlds sort of buff for the sapient masonry, and I think it's an honest shame that I don't have a remotely decent baltoy at the moment. Oh, well.