Whom should i rank up to R3? For uncollected?
If you're looking specifically at the Collector fight OML with Colossus synergy would be much better. The synergy let's you start with a bar of power so you'll get to sp2 quick for a big damage burst that X23 can't provide. If he's working properly (bleed reduction fixed) he can help on some other nodes and with his regen he'll be a sustainable lane clearer for you.
For UC, OML is far better because he’s immune to AAR. Your AA won’t reduce in time against collector.