I count both I-95 and the Merritt Parkway as a single unit. Milford to Stamford is the worst stretch in the state, even with 2 major highways covering the same area at times only a few miles apart. I'd rate the Q Bridge in New Haven as 2nd worst, you can spend years there. Luckily when I have to go that way I'm going the opposite of the traffic flow, so I don't get caught in the worst. Danbury to Hartford on I-84 is 3rd because there are only a couple of spots that tie up.
Route 8 isn't so bad, almost all of that entire road from Bridgeport to the Massachusetts line is glorious. The only bad stretch is the Commodore Hull bridge in Derby and you can take comfort from knowing that once you get through the rest is smooth sailing.