For anyone else on Earth, the first choice for a second language is English.
For an English speaker, well, we're pretty much stuck. English is the lingua franca of international trade, of diplomacy and of science. It's where French was in the 17th and 18th centuries.
I've heard that Czech is remarkably concise, that Loglan is logical to a fault and Esperanto still has its followers.
Which language would you choose if you had to learn a new one?
I can get by in German and Bahasa Indonesian. The lingua franca of the future is Chinese. They produce about four times the number of engineers than us. They may, or may not, be the same quality. I don’t know. The have 30 million youth studying classical piano. We have 30 million aspiring rap singers. We have them beat in the diversity department. Repeat after me: Diversity is our strength. And remember 2+ 2 = 5 or so they say.