10. Girls Night Out (season 4, episode 5)
Before we begin, I just want to say that I never had an issue with "Girls Night Out". It was a pretty enjoyable filler episode that also added to the overall emotional arc of the season, which was Barry and Iris' wedding. However, it was also a bit pointless, and full of that in-your-face humor that The Flash was beginning to rely on just a little too much, so I can see why it rubbed some people up the wrong way.
The episode saw Iris enjoy her bachelorette party with Cecile, Caitlin, and Arrow's Felicity Smoak, while Barry went out on his bachelor party with Cisco, Joe, and Ralph. The former party ended up getting in trouble with Amunet Black, who wanted to get even with Killer Frost - who resurfaced to save the day. It was a one-and-done adventure, but a serviceable one at that, and it has gone down in history for Barry's drunken "I'm The Flash!" moment.
What's pretty funny is that "Girls Night Out" has aged remarkably well. No it hasn't particularly gotten any better, but it is of a significantly higher standard than the low points that came from later seasons. Sure, there are shortcomings, but there are some solid story beats, hilarious moments, and a strong central plot that never just goes through the motions in the way that many of the lacklustre episodes in the final few seasons did.