This kind of question is like asking whether red is better than green :-)
Instead of ranking what is "natural" I suggest looking at the qualities of the instruments and trying to understand how someone tries to exploit those qualities to perform.
Piano cannot bend pitch or sustain long notes or crescendo after a key is struck, but it can play ...
I find the problem relies on the fact that you have not enough vocabulary.
Improvisation is a language. As such, you need to know words that will allow you to form sentences. A word is the equivalent of a short phrase. A sentence could be a lick that is composed of several words (short phrases).
Then you need to know your context. The context is where the ...
If you mean to say that singing is the "best" way to improvise melodies, I'll present a different point of view. Being able to play instruments provides structured ways to reason about music that you most probably don't have if you cannot play any instrument. And being able to read and write musical notation provides another such framework, or ...
I'm going to say "Keep It Simple" (and no, you are not "Stupid" so it's not KISS). For the moment, forget about scales and work on and fiddle around with arpeggios. When you do that you will hear "Hey! That sounds familiar!" because something in the arpeggio is used in a tune you know.
Also, learn really simple stuff like the ...
Here's another answer.
Trying speaking (not singing) a phrase that has emotion in it. For example
What a wonderful day!
I feel so sad.
Notice that when you speak, your voice will go up and down in pitch and the more dramatically you speak, the more the change.
Notice that you naturally emphasise certain words and syllables more than others.
Notice that ...