@Mr_NutterButter German is probably the easiest for an English speaker to learn I'd reckon, with Russian definitely being the hardest. However, it's probably the least useful if you live in America. Russian would be the most useful of the three imo, whereas French sits as the middle ground between both factors.

Russian is useful to learn because God forbid something happens where we either fall to Russia or have to gtfo of America and go to Russia. There's also huge parts of the internet that are Russian, and if you're going for a cybersecurity job or something in that sphere, knowing Russian would greatly enhance your career outlook.

French is just more useful than German because there's more people who speak it in America.

Another really useful one is mandarin Chinese, for the same reasons as Russian and also for business purposes.