based on my experience it would be Korean and I don't know why, I guess it's about interest and the reason why you want to learn the language.
Since I was young my grandpa start to teach me Spanish I use it for a while, I was quite good actually but when he died and I don't have anyone to converse with using the language I kindah forgot, of course I still know the basics but not as good as I once once.
I learn french too (my foreign language subject) but for one year all I know is the words of introduction we were forced to memorize.
I learn Korean for six months (speaking only) and after six month I am quite good and until now I can still converse, of course I have the accent but I am understood. :)