I would either connect via LAX to Alitalia (to start your Italian experience early) or go via CDG/AMS on DL/AF/KL. Splitting up the trip with a long US domestic flight really hurts you in the jet lag, comfort and sleep department.

I prefer connecting in CDG over AMS because the aircraft serving the airport tend to be better, the AF lounges are better, and DL caters better wine onboard for CDG flights than for AMS. But the airport is a bit confusing and there can be lots of walking, so if you are mobility impaired you may prefer AMS.

Alitalia will be a polarizing option. They are in serious financial trouble and have been knocking on bankruptcy’s door for a while now. But their business class seat is excellent, and I really like how they do their meal service. And arriving directly into FCO would be nice. I probably would only consider them if traveling in the next six months or so, though. The point may be moot because I rarely see competitive pricing on their flights out of Seattle...

For DL flights the onboard service is similar on all transatlantic flights, except LHR/CDG get better wine selections. As far as seat goes, A350 > A330 > 777 >> 767, in my book.

AF onboard is excellent all around, but only if you get an aircraft with their BEST seat (new 777s and 787s). I am not sure what they are currently serving SEA or YVR with; it varies day to day so you will want to closely examine the seat map for any given flight you are considering.

Avoid KLM entirely TATL unless onboard a 787.

AF only if 77, not the 380.
KL only if 74 (seats are new, but 2-2 upstairs and only a few are direct aisle downstairs) or 78. KL 77 and 330 are 2-2-2 in WBC.

KL 777/747 don’t have direct aisle access, which is important to a lot of folks. If traveling as a couple, these options can be a little more attractive.

Are you going on a paid ticket or an award ticket?
If on a paid ticket obviously you have access to all options, including partners with the only barrier being what you aren’t willing to pay or schedules you don’t want. If you’re using miles, you’re limited on what’s available with miles.

As others have mentioned, all Delta D1 seats are not the same. Most find the A330 far superior to the 767. Additinally, you may want to look at SEA-AMS/CDG-FCO as that will give you the longer flight in J while avoiding a domestic hop in a domestic F product (not that intra-Europe in J is better but here I think it’s better to get the longer segment - the TATL segment - in J). That’s another common sentiment on FT - try to get the longer flight on the aircraft with D1 so you have more time to sleep and enjoy the product.

There’s also VS (Virgin Atlantic) via LHR out of SEA then a connection to AZ as an option. I have an award ticket for FCO-JFK via LHR on AZ/VS coming up soon so it’s bootable as a single ticket.

Customs clearance at FCO can be a nightmare. I prefer to connect through AMS, BRU or CDG, clear EU entry there, and connect to FCO

I’d also add that other tiny Italian airports can be even worse. I once spent almost an hour waiting for two border agents to process just one Ryanair flight worth of passengers at Perugia, for example...