If you're converting regularly, then whatever is highest earning (currently Karbo, I believe, check whattomine to be sure).

Long term I suggest Electroneum. I've been following it for a while now and I like it and its dev team, and it's one of the few coins that has put in real work to be a usable currency. YMMV, lots of people on here will tell you ETN is a shit coin, and they may very well be right, just offering my two cents.