Splitting Season 5 into two subsets makes this a flawed poll... unless you were to do the same for the other seasons and even then it's a completely arbitrary design. For example, I no longer remember when Xin Zhao released, but everyone knows he was terrifying for quite awhile, meaning you could split the season he was created into pre-Xin Zhao and post Xin Zhao and every other season has something of this sort as well.
Anyway, I chose season 5. The reason for this being it was incredibly diverse, hosted many different metas (cinderhaulk, devourer, juggernaut update) and always had this feeling of being "fresh." It had one of the most diverse rosters, at least when it came to Solo Que, and pretty much anything could be played, to a certain degree. Sure, when Darius was first reworked he was bullshit, devourer got really annoying, and Cinderhulk meta was out of whack, but it was FUN and pretty much anything could go. The jungle was the most balanced it's ever probably been minus the devourer meta.
Considering the fact that I mained Udyr for the majority of season 5 along with Rengar, this is also a big reason as to why. After patch 6.2, Rengar was neutered, and after Ruinglave Udyr was neutered and I never liked the AP playstyle anyway. Mordekaiser had yet to be reworked into the buggy mess he currently is, at least for the beginning of it. It was also the first season I played competitively (before I was ranking up my account).
Thus far, I'm not really impressed with season 7, but at least I can see Riot putting in effort to fix the complete mess that season 6 was. I hated Dynamic Que... I was a pure solo player for all of season 6, I hit diamond two or three times, but I still hated the concept of it.