The obvious no. 1 on my motorsports bucket list is visiting Le Mans 24h. Just the greatest single race in the World.
Then again, I want to visit an F1 race some day, unless they completely destroy the series. The race I'd like to visit the most would be Silverstone. Of course, Monaco is the blue riband race of F1 but it feels so posh. Britain just feels like the home country and visiting the British GP would be cool. Another GP I'd like to visit more than Monaco is the Italian GP at Monza. I assume Monza has a great atmosphere, even though I'm not even a Ferrari fan. And of course, Monaco would be cool to visit, yet not the one I want to visit the most.
As for other series, Indy 500 would be cool to visit.