When you're in a baseline rally, it's best to hit crosscourt.
The reason?
The ball will travel over the middle of the net, which is the lowest point. The ball also has farther to travel on the diagonal so you have more margin for error.
Another good baseline shot is right up the middle of the court. It's a nice, safe shot and it doesn't give your opponent any angle to play with on the return. Chances are he or she will hit it right back to you!
Think of a baseline rally like two boxers sparring. You are hitting the balls back and forth waiting for an opening that you can exploit either with a clean winner or an approach shot.