You can make the argument that he was underused in the season but when Wally West was on screen he was refreshing. He brought that same level of enthusiasm that Barry Allen had in Season One when he first received his blessing from the speed force. In Season 3 audiences were able to re-live those moments and root for the young speedster. His powers and his healthy competitiveness with The Flash was a welcomed addition.
2) Iris Death Twist: (Spoilers)
The entire season was geared around the death of Iris West and it gave the episodes a little more desperation and danger. So in the season finale to find out that the character H.R. switched with Iris to die in her place was rather satisfying, particularly because the H.R. character was searching all season long for his purpose and he found it in the most amazing way. It was a nice consolation prize to seeing Iris actually die for most fans and the show delivered this “twist” nicely.
1) Savitar/Barry: (Spoilers)
The evil doppelganger angle has been done constantly in the Flash series but this was different--it felt different. Team Flash had worked tirelessly to stop this villain Savitar only to find out that their beloved hero was Savitar. It was a spine-tingling reveal that raised the stakes for the show all the way up to the season finale. Barry/Flash is such a light character so to see him fall victim to the darkness made the character feel more human than it has ever felt before.