Fred Norman threw both a screw ball and a curve ball. Not sure who else, but I can’t imagine he would have been the only one who did.The arm motion for a screwball is totally opposite from a curveball so I'm not sure I've ever heard of a pitcher having both. Screwball is very rare by itself. Now, as another poster stated, certainly a two seamer, slider, sinker all have different spins and make the baseball move a different direction, even in rare cases, the opposite of a curve.
My OP was more of returning a serve in play vs. hitting a baseball in play. Yes, getting a racquet on a 125 mph serve is most likely easier but hitting it over the net, inside the court boundaries is much, much more difficult. Same for baseball - hitting a fair ball against a 95+ mph pitch a lot different than getting a foul tip.
As for those who say 'baseball and it's not even close', I'm guessing none of you have ever tried to return the serve of a really good tennis player. Because, I have and it's a daunting task to say the least. A big serve just explodes off the racquet and, in most cases, you need to move both feet into position in order to get a swing at it. A lot of moving parts within a fraction of a second not to mention a totally different swing if it's on the forehand or backhand side. Neither is easy and I wouldn't argue for or against either one being more difficult. But, 'not even close' is showing a lack of knowledge of how hard a tennis serve can be to return, imo.
As far as the question goes, I would say it’s much harder to hit a baseball. I played both for years and it was certainly harder for me to hit a good pitcher than it was to return serve against a good server.