These are your options for serves:
1a) The serve that gives your opponent the most trouble
1b) The serve that gives you the best 3rd ball to attack
2)The serve your opponent least expects.
These are the main tactics I use when serving. In execution, it can look vastly different depending on my opponent. that is because everyone has their own strengths and weaknesses.
A good way to test for 1a) is to give your opponent as fast, long, underspin serve right into the elbow of their playing arm. How did they respond? was there good footwork on the receive? did they try and loop or push back? use their backhand or forehand?
Here's another couple of basic concepts to understand:
1) if you serve long, expect the return to come long most of the time (adjust your footwork on the third ball accordingly). Likewise, if you serve short, the return is most likely to come back short
2) if you serve to the forehand, most likely the return will come back across the table diagonally back to your own forehand. likewise, if you serve to your opponents backhand, the easiest return for your opponent is back diagonal across the table to your backhand.
With these two basic concepts, you can use the serve that you need to set up your best third ball attack. Just remember, a high likelihood doesnt mean it would happen every time, so you need to need to be prepared for the alternatives (like a return down the line instead of diagonal, or a shorter serve getting flipped back fast). The best serve returners are the ones that can return the ball where the serve LEAST expects it. however, depending on your level, chances are your opponents are going to do the MOST LIKELY serve return a MAJORITY of the time.