Speaking of the artwork in Aquaman stories, the character who underwent the most change over time was Aqualad. Here's how Ramona Fradon drew Aqualad:
With freckles!
When Nick Cardy began his run on the Aquaman feature stories with Showcase 31, his take on Aqualad was initially very consistent with Fradon's:
Eventually though Cardy rid Aqualad's face of freckles and darkened his hair from brown to black while making it much wavier:
As he grew older, Aqualad got a rather thick-thighed not entirely svelte girlfriend Tula in issue #33:
The changes in the way Cardy rendered Aqualad became particularly evident in the Teen Titans title:
When Jim Aparo took over the art duties in Aquaman 40, Aqualad was clearly older and had, in modern day parlance, acquired a grittier edge:
Here from issue #43:
Aqualad's epic battle against the Bugala took place in Aquaman #44-48.
To my eyes it was under Jim Aparo's rendering that Aqualad really came into his own.