I've played so many over the years, but I think the sports games I've derived the most total enjoyment from were Tennis, Baseball, and Golf for the original Nintendo Game Boy.
My parents bought Game Boys for my siblings and me in 1991 to keep us occupied on the long flight during our move from Japan back to the US (remember that, @Rusty and @Anythiene?). It was the height of extravagance for us, and a complete surprise — it hadn't even occurred to me to beg for a Game Boy because I didn't think there was any chance I'd get one. Thanks Mom (@Geek2Nurse)!
I spent countless hours playing Tennis, Baseball, and Golf on that Game Boy. They were super simple games, but endless fun.