I am happy to see many others with the same answer because, for whatever reason, it seems to be a controversial topic and I didn't one to be the first one to (unintentionally) stir the pot...but, for me: VR.
Now, I'll confess, if your wheel and VR broke on the same day and you only had budget for one, you'd replace the wheel first (duh). The VR experience is the icing on top of a large (and expensive!) cake: a gaming PC, a sim you like, decent wheel and pedals, comfortable seating arrangement, etc. But VR has just taken the sim racing experience into a different stratosphere for me. If I was on a fixed budget and choosing between passable gear + VR or nice gear and no VR...give me passable gear + VR 10 times out of 10, please.
Things that I have observed VR to do significantly better than a screen:
1) Ability to sense both relative and absolute speed
2) Ability to perceive both relative and absolute depth and distance
3) Ability to perceive shape and camber of corners
4) Ability to gauge car's position relative to the track
5) Simulating the sensation of the car "following your eyes"
6) Ability to track "situational awareness" when on track with other cars
7) Ability to practice good real-life mirror habits (quick look left/look right on any straight)
What did I have to give up for this? Slightly better resolution. After a few laps in VR for me, that became more like "it will be cool when VR has better resolution". VR makes the driving experience so much more engaging from an overall sensory perspective, I don't have the time (or desire) to nit-pick that I can't quite read that billboard or those trees look grainy or what have you. I have no evidence to back this up, this could just be my brain filling in the gaps for me, but the crispness seems to have improved since I first got VR in Oct 2017. Better drivers/software and graphics cards perhaps? I certainly haven't switched headsets or (knowingly) cranked up any settings. It might be a combination of the two - handling of VR improving plus my brain doing some compensation.
Screen door effect is very real but, honestly, I don't notice it anymore. I'm being serious, I have to work really hard to see that it's there, my brain filters it out as noise.
Motion sickness has never occurred for me. I think that may just be lucky biology/physiology, but my understanding is that very few VR users actually report motion sickness.
Bottom line, VR is a revolutionary step in the sim racing experience. I suspect if you did a well controlled study, you would find the leap in satisfaction from no VR to VR would, for most people, outpace every other enhancement (from upgraded wheel, to triples, to load cells) except for no wheel to wheel. No motion to motion might also give VR a run for it's money, but motion is also several times more expensive than VR (last I checked, at least) so you have that to factor in. And I would personally have to budget for GOOD motion simulation. I tried one out last year that was so crude and clumsy that I found it to be distracting. A simple Buttkicker would have blown that particular "motion rig" out of the water for my tastes.
In conclusion, I like VR and encourage anyone who can to try it for themselves.