Well, Itachi was without a doubt the best Genin of his generation.
In current Naruto before the 2 year jump, I'd go for Neji, Gaara or Rock Lee in terms of the most powerful Genin
(Rock Lee should just forget about that stupid lotus technique, its too risky. If he had just kept his first gate open,
he would probably have pounded Gaara into the ground). They're the best in their fields, but each is weak against
one other. Gaara can't keep up with Rock Lee, Rock Lee is too vulnerable to Neji and Neji can't touch
Gaara.
My personal favourite Genin is Shikamaru though. That super genius will analyse, trick, bluff and confuse enemies with
his sheer brainpower, alternating between checkmating his opponent and daydreaming about the clouds. lol.