S5 definitely has it's problems but after discussing about basic powers (and prior conversations about the BTS drama), I switched my vote to "somewhere in S2". That "somewhere" being as soon as it began. Let me go over my data and NOT opinions:
S2 began the "sexy" outfits. Or, at least, perpetuated it.
S2 is a constant struggle between Constance M. Burge and everyone else who didn't share her vision.
S2 MASSIVELY changes the personality of BOTH Prue AND Piper. Piper is crankier than usual, and Prue is less bossy. ONLY Phoebe remained consistent. (I get folks like less bossy Prue, but that's not my point)
SS2E2 "Morality Bites" gives such a high expectation of what their basic powers COULD advance like but (also) promises some sort of lethal power Phoebe could have but never fully fleshed it out. S6 tried with Empathy but didn't quite hit the mark. This is (also) the ONLY episode where Phoebe has more than one premonition which leads me to my next point.
S2 MASSIVELY cuts down on Phoebe premonitions. S1, she'd be having premonitions left and right even repeated them. S2 doesn't do this. Number of S1 episodes where Phoebe either only has 1 premonition or no premonition at all:
* = indicates NO premonition
*S1E5 "Dream Sorcerer"
S1E6 "The Wedfung from Hell"
S1E7 "The Fourth Sister"
S1E10 "Wicca Envy"
S1E13 "From Fear to Eternity"
*S1E14 "Secrets & Guys"
*S1E17 "That 70s Episode"
S1E18 "When Bad Warlocks Go Good"
S1E19 "Out of Sight"
*S1E21 "Love Hurts" (but, Prue gets TWO premonitions during their power swap)
Number of S1 episodes where Phoebe is having more than one premonition:
* = Contains Non-Visual Premonitions
S1E1 "Something Wicca This Way Comes"
S1E2 "I've Got You Under My Skin"
S1E3 "Thank You for Not Morphing"
*S1E4 "Dead Man Dating"
S1E8 "The Truth is Out There and it Hurts"
S1E9 "The Witch is Back"
S1E11 "Feats of Clay"
S1E12 "The Wendigo"
S1E15 "Is There a Woogy, in the House?"
S1E16 "Which Prue is it, Anyway?"
S1E20 "The Power of Two"
S1E22 "Deja Vu, all Over, Again"
It's fairly evenly split but by the time we hit S2, she's only having 1 single premonition and that's it.
S2E9 "Ms. Hellfire" gives Prue a secondary power and lowers the amount of episodes where JUST their basic powers are used and further alongs ALL sisters getting secondary powers in S3. But, by the end of S4, the show refuses to balance out those powers the way S2-S4 did.
Now, to my OPINION, I always wondered why it took me awhile to appreciate S2, and this is the reason why. It's NOTHING like S1 nor what Burge initially envisioned and changes A LOT of things. The only way for me to like it is ANY of them using ANY of their abilities, TOGETHER in the same episode. This is where S2-S4 shines. Then, S5 even dumps on that.
Just wanted to share this information. Do with it what you will.