I´ve been thinking about it for a while. From my point of view, if my dreams were to be true, it would be latin (I have been learning it for two years now), but I know thats not possible. Even with ancient greek.
I would support latin or greek over any modern european language because Rome and the greek cities were the based of the west, specially latin was the language of the cultured elites until the XIX century. Moreover they have no nation today that holds them as a native language so they would be more neutral.
Given that it is not probable to revive them, the other major languages in Europe would be French, German or Spanish. In mainland Europe german and french have more speakers than spanish, but if we include the potential of the spanish speakers throughout the world spanish would be a good option for lingua franca i think, the problem is that Spain nowadays is even a gretaer meme than France and Germany but thats nother topic.
Personally as someone who enjoys learning languages, I hate english due to being so simple. I truly believe that it undermines the languages with which it has contact, for example swedish is being replaced by it, (and arab of course). And of course, I quite hate england, so whathever to weak its influence is good for me.