XXL went over this topic once, about how Pac both is and isn't the G.O.A.T.
Basically Pac was never about complexity, he was all about emotion, meaning, depth, and charisma... stuff like that. The funny thing is, most rappers blow you away with two lines, because of how they rhyme ("They make me want to trip with this gauge/ And you suckas will be 'Gone in 60 Seconds'/ Quicker than Nicholas Cage" - Crooked I), but Pac could blow you away with just one line, because of how deep it is... you didn't even need to hear the next line or even check if it rhymed, it just grabs you, and you're just like, "Daaamn that was dope." And just because Pac didn't use a particular style or rhyme pattern doesn't mean he was ignorant... he knew all about iambic pentameter and stuff like that.
But toward the end, his complexity was increasing, just look at his verses from "First 2 Bomb":
"Tell me, baby, what's your frequency
I see you making proposals full of verbal indecencies when you're meeting me"
"Two Glocks is full of ammo, My army fatigues
Ready for battle, lyrical commando, let's get it on
Tell me, nigga how much you can handle
Banging on wax, I turn this track into a roman candle"
"First to bomb, first is the calm, then the panic
Soon as my niggas break, we earthquake the whole planet"
"Life-long committed, I write songs and spit it"
I still remember how the day after this song was leaked from Mike Dean's website, I was still blown away. In this song, Pac still had all of his strengths... his depth, his passion, and ability to combine different topics (the song starts off being toward a girl in the first two lines as I typed above, then it becomes some war shit). In addition, his rhymes were becoming multi-syllabic (meaning not just the last syllable of every line) and rhyming in different parts of the line, plus he was throwing some imagery, metaphors, and similes. So he was definitely getting better.