One of the best ways to dramatically improve your game is to develop serves that lead to developing other aspects of your game. The stronger serves and developing other aspects of your game lead to a higher level of play, meaning you get to play stronger opponents, which leads to further improvement. The stronger opponents force you to continue to develop the serves and other aspects of your game, leading to a snowballing effect that can, over time, dramatically improve your level of play.
I'm going to use my experience with this as an example. During my first few years I focused a lot on developing my serves. The result was I would get lots of relatively weak returns to attack. This developed both my attack and my footwork, which made me better both on my serve as well as in rallies. Because I became dominant on my serve and my attacks and footwork improved because of my serve, my overall level improved, and I began playing stronger players. Going up against better players pushed me to even higher levels. I was forced to improve my serve even more to keep them effective against stronger players. Since I had developed a strong serve and attack, as I played better players who received my serves better, I was forced to continue to improve my attack and footwork. The result? By developing my serves early on it snowballed my development. I was able to go from beginner to 1900 in about two and a half years, and 2100 in five. (With rating inflation, that's more like 2000 and 2200.)
So what does this mean for you? Develop strong serves that allow you to consistently serve and attack. Such serves don't have to give you easy pop-ups; it's sufficient that they consistently give you balls you can attack, while forcing a number of "free" points as opponents make mistakes (often from trying to receive so you can't attack). Develop both third-ball serves (that allow you to consistently serve and attack) and a few "trick" serves (that give you free points if not overused - most such serves become ineffective after a few usages). Learn to serve with great spin and deception (including no-spin serves that look spinny), both short and long, to all parts of the table, with different serving motions. If you aren't sure how to do these serves, talk to a coach or top player.