I can only think of only three flashbacks that were worth the time that they occupied in the series

1> Haku's history
2> Gaara's history
3> Naruto in ep 78 realizing what 'true strength' is. However, it wasn't the content that was so good, but more of a very fast scene sequence that accumulated, basically, the whole series, all the way from episode 1.

Though they may be obvious, they are also integral to the whole series and the arcs that they were involved in. A lot of other flashbacks were just useless, and just ended up being time fillers, especially those in the preliminary fighting matches. My god. Those were painful to sit through