Witch one checks first if deadpool rolls a 4 or 5 for RR

Interesting interaction, this one. Here's the text for Ridiculous Regeneration:

And here's Invulnerable:

The section in bold states that the effects of Ridiculous Regeneration take place instead of damage being dealt. As such, there is never a 'damage dealt' to be reduced by 2 by Invulnerable. On a 4-5, in place of damage dealt, Deadpool simply takes 1 damage. As this is damage taken, rather than damage dealt, it is not effected by Invulnerable at this stage, either. In short, damage reducing game effects have no effect when combined with Ridiculous Regeneration.

...unless that damage being dealt to Deadpool (before RR) is 2 or less... then if he had Inv, it wouldn't activate RR because he's not being dealt damage.

Incorrect, invulnerability would be triggered when you are dealt damage, ridiculous regeneration triggers when you would be dealt damage from an attack. Ridiculous regeneration always triggers first, so if deadpool is attacked while possessing ridiculous regeneration the step when it would be reduced is skipped completely.

Incorrect. Regardless of the damage dealt, be it 1 or 100, Ridiculous Regeneration will activate instead of a damage reducer. The reason being is RR activates, as Pineappolis said above, when Deadpool would be dealt damage. Damage reducers only activate when a character is dealt damage.

That makes sense, but how about this. At the point when RR deals damage, then the invuln would kick in. RR doesn't say the damage dealt by RR is unavoidable or penetrating, so the invuln would be used to reduce it.

That doesn't work either, because RR doesn't cause Deadpool to be dealt damage after the roll, he takes damage. Taking damage is the act of turning the dial, which happens after damage reducers.

Nope. You cannot stack RR with a damage Reducer. This has already been ruled upon by the Rules Deputies

A damage reducer + RR does nothing as RR overrides it.

Damage dealt and damage taken are separate things.
Ridiculous Regeneration rolls grant Damage Taken ("takes"). Damage Reducers affected Damage Dealt.

oh well, that could have been cool.

Ah, dang, good point... didn't read the power very clearly in my attempt to answer quickly. I should stop trying to rush through questions while at work.

Instead of a reducer, get DP some Super Senses, or Shape Change.