The problem with this sort of question is it's a bit of a "sweet shop" style scenario, which do you pick first!
I think at the top would be Indycar, simply because it's building momentum again, grids are good and very few of the tracks are officially included in sims. It's the right time for it to have it's own title.
Then IMSA, again because it's largely ignored. Would be so great to have a full season, day/night, weather. I'm still surprised that DPi hasn't officially featured in any PC base sims yet. I've yet to try the AMS mod, looks good though.
I'm pretty glad that ACC has appeared, it shows any prospective studios that there is a market for a single championship based game/sim still.
A WEC/ELMS based sim. WEC has the big factory teams, but ELMS has the larger grids and possibly is easier to re-create by a studio.
It's a shame that Sector3 can no longer produce the DTM seasons, they did a very job of it and the experiences were also pretty good too (just a shame they were dropped). I still race on them now.
In fairness there are dozens of championships that deserve their own title (I'm a huge fan of SuperGT as well), so I'm not surprised that there have been so many replies.