Season 6 is actually a great season. Trust me. If you feel the need to stop, wait for S7. THAT is when the tone AND lighting go south. (S7 is dark both metaphorically and physically.) Of course, you may feel differently if you keep going. But, for me, that is when the show started having one character eat the show and become a Mary Sue. (I'm looking at YOU, Abby!)
However, the show - after S6 -basically becomes uneven. Some parts are great! But the lows are pretty low. (Season 8 also sees Susan Lewis return, but with the shitty writing she got, she may as well have been a new character.)
One season I completely recommend is the final one, S15. Bunches of returns from the classic cast, and it was just a great wrap up. And Angela Bassett, for coming in so late, was a good addition.