How would you rank the different racing series? I don't mean your personal preference but based on the quality of drivers and manufacturers/teams and the prestige of the series.
I leave rallying and bikes off from my list as it would be comparing apples with oranges.
#1 F1 - The obvious no. 1 and will remain unless they completely lose the plot or something revolutionary happen.
#2 NASCAR Sprint Cup Series. - If a racing driver's dream isn't to make F1, then it's most likely NSCS. And obviously the series with second-biggest following.
#3 WEC - Starting to surpass F1 (or already has) as the most technologically advanced racing series. And is nowadays the no. 1 destination for drivers when they're finished with / never made F1.
#4 Verizon IndyCar Series - A shadow of the heyday of CART but still the only open-wheel series besides F1 with prestige and the Indy 500 is possibly the greatest race on Earth.
After that it gets harder to rank them. I'd give #5 for DTM as it seems like the only touring/sports car series where you can still get to F1. But otherwise it's hard to compare junior open-wheel series and touring/sports car series, the first ones are for drivers aiming to F1, the latter ones for those who never made it or couldn't stay there. A great F3 driver might not be ready to succeed in Super GT, WTCC, or something but he might be a future F1 WDC.