Guys, Sticky Hold and Liquid Ooze are our best choices here, for several reasons.
Sticky Hold:
1) From purely a flavor-perspective, it makes sense. Crucibelle is a gooey, sticky Pokemon, and I could see it being possessive with it's belongings, wanting to keep them secure in its pot.
2) It has absolutely ZERO chance to outclass it's primary abilities. For one, Crucibelle is most likely going to be used with it's megastone attached, making Sticky Hold absolutely useless. For two, no one in their right mind would elect to use Sticky Hold on non-mega Crucibelle over Regenerator; the pay off would be minimal, with the only positive being able to hold onto leftovers, which is still heavily outclassed by Regenerator.
Liquid Ooze:
1) Again, as far as flavor is concerned, this one is really self-explanatory. It's a Pokemon made of ooze. No-brainer.
2) Also again, this has zero chance to outperform either of it's primary abilities. Only Grass-type Pokemon will be using Giga Drain or Leech Seed against this thing, but that's a moot point, since our Poison-type STAB attacks will scare most of them off anyways. And for those that aren't scared (i.e. MegaVenusaur) having Regenerator or Mold Breaker still makes more sense.
Either way, neither of these abilities is useful enough to lend themselves to even a niche use. They fit flavor perfectly, and will never be used competitively over Crucibelle's other abilities.