If anyone doubts baseball and hitting, just go find a batting machine and set it at 70mph and have at it. That's typically slower than the slowest curve balls the pros throw. When you've become proficient at that, go find a college pitcher to toss you a few.
When you're done missing everything, remember....that was a college pitcher. The pros will make you cry...or eat a lot of dirt.
Tennis. Granted, hitting a baseball is very hard. Hit 3 out of 10 and you are a legend. Hit 2 out of 10 and you won't stay in the professional ranks. A baseball bat is much smaller than a tennis racket and the last time I checked baseballs are very hard.
That stated, Tennis serves are about 25% faster than a 95mph fastball. But instead of hitting a ball passing through a knee-to-waist zone 18 inches wide a tennis player needs to be able to return the service both forehand and backhand in a much larger zone off the bounce (which may take the ball out of the zone) and accurately return to a much more confined valid return space (i.e. you cannot just drive the ball like in baseball). A master tennis server is going to be putting crazy side, top, and back spin on the balls, too, which make it react much like a baseball. And about that bounce, depending on the spin and surface it will react differently. The ball is going to travel differently on grass, clay, etc. The technique is also quite different in regards to foot work. Baseball players are always playing with the pitcher on the rubber tossing over dirt. Oh yeah, a tennis server can move and setup different attack angles.
Yes, the racket head is bigger in tennis than a baseball bat. But the sweet spot isn't as big as the racket face. And statistically I bet tennis players get their racket on the ball more often than a baseball player gets the bat on the ball, including fouls. But this isn't so cut and dry once you consider baseball players rarely bunt (high percentage bat on ball) and many tennis returns are essentially a "bunt" and just an effort to get the ball into play.
Of course serving/return is just one part of tennis. In baseball a hitter can stink at defense and either be a liability in the field and be tossed into a position with minimal ball handling (e.g. right field) or moved to DH and play no defense at all! Ditto pitchers. Most cannot hit and their fielding is poor. They are paid to master one aspect of baseball, and that aspect only. A whole league (AL) doesn't ask them to hit at all!
There are no designated hitters in tennis. There are not "servers" who take no part in defense or hitting. In tennis you must be able to serve, return service, and volley. Obviously, not all tennis pros are equals at all aspect of the game. That is the point: very few can master every aspect. But unlike baseball there is no free pass for stinking. If you serve great and volley great but cannot return a professional service you won't be a pro.
So while baseball has its fair share of very slow, even fat, players who do one thing well (pitch or hit) the skill to master tennis is much, much higher. And of course there is the entire conditioning aspect where tennis is surprisingly very taxing.
The pool of tennis pros (masters) is also much smaller than baseball where in America you have about 30 teams with 25 active players on the roster with another 15 or so designated. Counting "pros" as the pool of mastered players baseball has well over 700 active at any one time. Tennis has much fewer (64 at the biggest tournaments). In this sense Tennis has the confined field like golf but requires the skills of a more demanding sport. Unlike golf in tennis there is no "lucky day" where a complete hack can get below par. That is because in tennis you always have an opponent that can neutralize your effort. It takes little skill to hit a nice golf shot; it takes a lot of skill to hit a nice golf shot regularly. In tennis you see a top tier who based on skill regularly separate themselves; in golf you regularly see less skilled players win. To me that says golf requires less skill to master (and puts into perspective how much better Tiger Woods was than the competition in his prime as he was able to overcome this variability).
I also think football is harder than people think. Where football gets dinged is there are 22 active players on the field but room for a lot of specialists (3rd receiver, 3rd down back, jump TE, nickel corner, kicker, punter, kick returner), and 53 players on each roster. A player need not master each position but just be good at 1. That said the combine workout warriors prove that athleticism alone won't make a player a good pro. Football is more than freak genetics. It requires a large degree of toughness and the ability in most positions to remain mentally focused while in pain, fending off contact from equally physical opponents, while maintaining a high degree of discipline, balance, and field awareness.
Hockey is another sport, like football, that requires a combination of mental skill, fine motor skills (skating, pick handling), but also a lot of grit to receive a lot of pain while remaining mentally strong and not compromising the finer skills.