In my world history class, I have been assigned to proven why the Silla Empire was the best dynasty for a debate. I've learned that they had a major factor in Korea's art as "Korea's Golden Kingdom", but is it significant enough to win a debate and explain it as a major contribution?
The Samguk Sagi and Samguk Yusa don't really go into why Silla was successful beyond "such and such a king was more virtuous than the kings of Baekje and Goguryo." I'd actually debate the source that claimed Silla was a major contributor to Korean art; generally scholars agree that Baekje had the most advanced and beautiful art. Also, its not entirely appropriate to call Silla an empire. In the history of East Asia the only real empire is China. They use the characters Di Guo (帝国）to refer to their natiion, and the characters Huang Di (皇帝）to refer to their leader. Tributary states like the Korean kingdoms used the character Wang （王）to refer to their leader, and the character Guo （国）to refer to their nation.
That being said, Silla's mountainous territory was much more defensible than either Baekje or Goguryo, and Silla's diplomats were evidently much more skilled. When Tang turned its eyes to Korea, Silla adroitly formed an alliance with them and worked together to crush Baekje and Goguryo.
Tang of course then betrayed Silla, but Silla was able to trap the Tang army on the peninsula and use their diplomats to sue for peace. In exchange, they became tributaries of Tang. Additionally, the buffer state of Balhae (Chinese: Bohai, 渤海）was propped up in what was formerly the northern territories of Goguryo.
I'd recommend reading up on the events in Peter Lee, et. al Sources of Korean Tradition: Volume 1. I hope this helps!