It depends on the level of proficiency you're going for. If you just wanna hit the ball in play I would say the serve is the easiest to learn. If you want to play confidently at the 3.0/3.5 level I would say the serve is more difficult to use correctly because there are so many variables and ways to hit it (flat,kick,slice).
A serve is easy to learn because it is simply done in a very logical way. In particular, the racket face is aligned early so that a swing mostly forward will impact the ball
with the racket face aimed in the side-to-side direction of the desired target - completely logical. However, this concept results in the lower performance Waiter's Tray technique that most active tennis players are using. A WT can hit a slice and flat serve. But I cannot see how a WT can hit a kick or top spin serve, as there is no effective way to cause the racket head to rise as it is hitting the ball.
The WT is not found in any of the top 100 ATP players I've ever looked at, probably all use the high level serving technique that uses the axial rotation of the upper arm for
internal shoulder rotation (ISR). The high level serve allows the racket to be rising before impact and at the same time the racket face to be orientated in the side-to-side direction of the desired serve. It can then do the kick serve. This racket face orientation is also changing direction very rapidly during impact in the side-to-side direction.
The above comments are based on video observations of the racket face impacting the ball. Observe the 3 frames - before, during and after impact. I have posted videos & Toly gifs of the high level slice and kick serves many times.
This video shows that the high level serve both 1) swinging forward and 2) at the same time, the racket face is also rotating in the side-to-side direction.
To single frame on Youtube, use the period & comma keys. In contrast, you don't see the side-to-side rotation in the WT technique.
If you think that a WT can hit a kick serve, please post a video showing the racket head motion that rises during impact to produce the component of top spin.