If you want to improve, then it's important you develop standard third-ball attack serves - serves that are difficult for opponents to attack and set you up to attack, but usually don't win the point outright except when the opponent is overly aggressive. These are usually short or half-long (where second bounce would be right about the far end-line, sometimes barely off). Most opponents will push them back long, allowing you to loop. More advanced opponents may try pushing short or flipping, but if the serve is done properly and with enough variation, it's tricky to stop those third-ball attacks. The importance for most players to develop their game around such serves cannot be overemphasized - not only do they set up the attack, but over time, they allow players to develop their third-ball attacks (like most top players), and so their attacks (including their footwork to position themselves for it) get better and Better and BETTER. Let us call this the Yin.
But there is also the Yang. If you only do third-ball serves, you not only are giving up "free" points, but you are also making things predictable and therefore easier for your opponent. So you should also develop tricky long serves that, if used sporadically, catch the opponent off guard and give you these "free" points, either by outright misses or weak returns. If overused, such serves are susceptible to strong attacks, which is why they should be used sparingly. But if the opponent has to guard against them, then he is less ready to make effective returns of your normal third-ball serves.
The most common third-ball serves (Yin) are short or half-long with backspin (often combined with sidespin) or no-spin. (To serve no-spin, use the same big spin motion you'd use when serving with spin, but contact the ball near the slow-moving handle.) As long as these serves are very low to the net, they will usually set up an attack. You can also serve short topspin or sidespin-topspin as a variation.
The most common tricky deep serves (Yang) are big, breaking sidespins that go deep on the table, often breaking into the wide corners, though they can also break the other way. Another is fast no-spin, which can catch opponents off guard, especially if done right at the playing elbow, between the forehand and backhand. The more you do these types of serves, the more you develop an instinct for when to use them.
Having said all of the above, tricky deep serves can dominate up to a somewhat high level. Even 2000 players struggle returning these serves, if done at a high level and not overusing any one of them. Relying too much on these serves can give a lot of success to a certain extent, but they are limiting, because opponents get used to them, because higher-level players have less problems with them, and because they do not lead to your developing your own game - you instead rely on opponents missing.
But the other extreme, relying completely on third-ball serves, while maximizing your own improvement by developing your attack, is limiting as you are giving away "free" points you might have won, and making things easy for your opponent, who doesn't have to guard against these serves. Plus, if you rarely use tricky deep serves, you won't develop an instinct for when to use them.
So what's the solution? Find a balance. If your goal is to reach the high levels, focus on third-ball serves, but develop a variety of tricky deep serves as variations. If your goal isn't so high, and you are looking for a "quick fix" to improve your rating or ranking, then perhaps develop your game more around such tricky deep serves. (A small number of players have reached very high levels doing this, but it usually involves acrobatic counterlooping when the opponent loops their serve. Some defensive players also do this.)
It's all about finding the balance between Yin and Yang.