Well if you are refering to writing style as opposed to just
style which i would assume is delivery and such, my personal
favorite would have to be Sage Franics. He's an absolutely
amazing lyricist in my opinion. But it's pretty much different for
each person depending on what kind of hip-hop you listen to
or are into, because after all Hip-Hop is a huge genre.

As for your other questions, yes rhyming is just rhyming. Anyone
can do it, but putting substance into it, making it mean something
is entirely different. That is what truly takes skill and will set
a good emcee/writer apart from others. And styles evolve in
different ways in my opinion. One way is indeed through practice.
When someone first starts out they usually stick to using simple
schemes/lyrics until they have mastered it. Though once they've
developed to the point where they no longer need to rely on
such simplicity they may move to more intricate rhyme schemes
and lyrics to better convey their skill and mastery. On the other
hand some people just choose to utilize the style they feel most
comfortable with or a style influenced by other emcees they look
up to or have grown up on. But that's kind of frowned upon, so
I would encourage you to find your own unique style or way of
conveying what you want to say in pieces, songs, etc. In time
you will learn or adopt the style that suits you best; everyone
does it just takes practice and dedication to your craft.

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chillin' like refrigerators