I had just the opposite experience last year. I rode from DWG to Matamoras and had little to no traffic going up 209. It must be the time of year and day of week that you are on the road as to whether or not you have to deal with traffic.

I will suggest US6 through Alleghany National Forest. You do have to climb, west to east is a very consistent climb. Once you start up it you just keep going at the same grade pretty much until you hit the top. None of this bouncing around with easier and harder grades, its consistent. It's a real nice climb. I was kinda dreading given the way to the day had went thus far and it ended up being the best part of the day by way far, rode from Meadville to Olean, NY that day. Some nice scenic areas around the lakes as well before you start up the climb. I've ridden most of northern PA now, just three or four counties up at the north end that I haven't been in so far on the bike, hopefully that will be taken care in the next week or two as I'm hoping to be leaving out tomorrow or Thursday to finish off all the northern NY counties and the rest of the counties in northern PA that I haven't been in yet on the bike.

I would have to say the most scenic part of PA I've seen thus far has been US6/Alleghany National Forest or PA443 SW to NE is another nice ride. It has the unique appearance that you are both in the flat farmland as well as in the mountains.

Like I said I haven't been in all of PA YET, but I have been on the bike in a fair share thus far given I've only been in PA on the bike twice...either going to or coming back from Ohio.