I would say... the one that motivates you the most! Then, it will be easier :)
French is closer to English so if you don't know a word, you try the english one with weird French pronunciation and it works quite often! But I think that it's difficult to get a good level because we have many exceptions, littles words lire "Je te l'ai dit" "Qu'est ce que tu en penses?". And one tricky thing is that it's not written as it's spoken. Many French don't write correct sentences because of that.
For German, many words are differents and they have more declension but they stick to the rule. And write as they speak. But I love German, and it's always so funny to put the verb at the end of the sentence :)
And Chinese has the best Grammar ever. But sadly, you have to learn other things to be able to speak....