This is all just my personal opinion. This is my advice to section hikers who want it.
These recommendations are roughly in order, but I did not number them because the order really depends on how logistically complex someone is willing to get, how far they want to hike, and how much elevation change they want to deal with.
All of Section J
The third most beautiful section and by far the easiest logistically. You can take a bus to/from either end of that section. Also, it has many exit points in case you need to get off trail. It has only one challenging stream crossing in the spring. If you can manage the logistics, I like to add part of section I to the trip, Stampede Pass to Snoqualmie Pass.
All of Section L
The top two most beautiful sections, but has logistical challenges. Access from Rainy Pass is easy but no public transportation. Access from Harts Pass will require a ride on a road that scares the st out of me. And thru hiking to Canada requires a special permit, else you need to backtrack the way you came. This section might be the easiest section in terms of elevation change (especially if you only hike between Grasshopper Pass and Hopkins Lake). This section is the furthest east of the Cascade Crest and is the most prone to wildfires and smoke issues so that is a reason to try to get it done early in the season. Also, water collection is not very frequent in this section. There are no challenging stream crossings in this section.
Part of Section H, Goat Rocks
The north part of Section H. It is the other in the top two most beautiful sections. At the south end it can be accessed remotely at Potato Hill, Midway, or Wallupt Lake. The north end is White Pass, a paved highway with a store and a hotel. This section includes Old Snowy, the highest point on the PCT in Washington (low route) and the highest point in both WA and OR (high route). The low route never melts (yet) and the high route melts out fairly early. I avoid this section until I know the high route is clear. The low route is just a short easy traverse in snow at the top of the Packwood Glacier. While slipping seems unlikely, those that have slipped have traveled a long way quickly and been seriously injured. Goat Rocks also includes Knifes Edge which is exactly what it sounds like. It's really no more dangerous than any other steep slope on the PCT, but feels otherwise because there are slopes on both sides of the trail. There are no challenging stream crossings in this section.
Part of Section I, Chinook Pass to Naches Pass
Middle third of Section I. This portion of the trail is fairly high and offers amazing long distance views of the surrounding volcanos. It's high, exposed, and is where water is the most scarce. There is plenty of water at both ends (Sheep Lake and Naches River) but has only two other sources between and both require a little off-trail searching. This section ends at the Mike Ulrich Cabin. There are no challenging stream crossings in this section.
Part of Section H, Williams Mine (Trout Lake) to Midway
(or just continue on thru Goat Rocks to White Pass)
This section involves climbing up Mt Adams to about 6,000 ft, then circling its western slope, then hiking past the northern lava flow to a cinder cone (potato hill).
Part of Section K, Stevens Pass to the Suiattle River
The southern half of Section K. It's difficult for me to give advice on this section because of its mix of pros and cons. The issue here is difficulty and lack of exit points. This section has the most elevation change, the steepest climb in the state (Milk Creek). This section receives the least trail maintenance so may have many blow-downs. The Kennedy Creek crossing is one of the three most challenging and possibly the most dangerous because of the size of the watershed that feeds it. More solitude is a big advantage to this section. Once you are in, you are committed to some miles, even if you take an exit rote. A huge pro for this section is the beauty of Glacier Peak. This section requires an investment in planning and effort, but that investment will pay off big in beauty and satisfaction. A cool alternate route is to enter/exit by boat at Lucern, WA and take the Lyman Lake route. This adds additional elevation gain but lets you enjoy Suiattle Pass and Miners Creek.
Part of Section I, White Pass to Chinook Pass
This section has two distinct halves; from White Pass to Fish Lake there is effectively no elevation gain after the first four miles, the forest is more open, and there are many lakes and ponds. North of Fish Lake you climb up to the Cascade ridgeline and its a ridge hike the rest of the way, with limited water until you get to Dewey Lakes. Then you climb again up to Naches Peak and Chinook Pass. There is only one easy creek crossing at the Fish Lake outlet. For consistency I just described the trail south to north, but this section has a clear elevation advantage going southbound instead.
"The Chelan Loop"
Both the northern and southern routes on the PCT to High Bridge (Stehekin) are just long river walks. If Stehekin interests you, but river walks do not, one logistically easy but physically challenging route is to park in Chelan and take the ferry in and out, starting your hike at High Bridge, and ending it at Holden, and catching the ferry from Lucerne back to Chelan.
Another variation on this is to start and end in Stehekin by taking the PCT north and east to Fireweed Camp, then taking the the High Camp route back to Stehekin. I recommend this direction because its easier and offers great views over Lake Chelan.
What to skip
Okay, don't get mad at me, this is just my opinion, but here's the sections I would skip if I had limited time to hike:
Section H - Columbia River to Williams Mine. (There is a fair argument to be made for Indian Heaven, its just lower in my personal ranking)
Section I - Naches Pass to Yakima Pass.
Section K - Suiattle River to High Bridge.
Section K - High Bridge to Rainy Pass.