The Mountiangate Century in Los Angeles!

I'd estimate 100 miles and 25,000 feet of climbing.

Actually I do not know if it still happens, I rather doubt it. But any Los Angeles resident can see the course. Just take the San Diego freeway South through Sepulveda pass. On hte way down look to your right, there is a road going up to the gated community of Mountiangate. Turns out it is just about a mile long. Yup 50 laps up and down and with the sharp turns it means yuo never hit the climb with speed. It looks to be at least 10% grade.

This started as an April fools joke and then people actually rode it.

BTW The Los Angeles/ Orange County area also had a contender for easiest, and of a similar nature. 25 laps of Mile Square park. Dead flat and on a bike path.