Season 5 was to me divided into three very distinct phases.
Early season 5 was a complete shitfest, this was the point in time where the only viable junglers were Lee Sin, J4 and Rek'sai. Game balance was generally in a pretty shitty place. This was marked from patch 5.1 (possibly also preseason) to 5.4.
Mid season 5 is generally what people mean when they say "S5 was my favourite season". This started in 5.5 with the Cinderhulk changes and continued through 5.16 with the Juggernaut rework, this comprised most of S5 and the meta was generally quite enjoyable once Riot toned back Cinderhulk a bit, it was probably the most diverse League has been since S2.
Late season 5 was marked by the Juggernaut reworks in patch 5.16 and continued until 5.21 with the end of Season 5. This period killed off most of the diversity of mid season 5 and forced way too many 'must pick' champs into the meta. It's also noteworthy that Riot pushed a really random Cinderhulk change that pretty much killed off tank junglers in this same patch, the only one left was Gragas and he built Echo's by this point.
It's important to remember this when you talk about Season 5, because I think a majority of people would agree that the meta between about 5.7 and 5.15 was pretty solid, certainly better than anything since.