you might consider the chinese dogwood cornus kousa and it's various named selections---late spring flowers and good fall color. c. "eddie's white wonder" is another dogwood that might be useful. there are lots of flowering cherries ("kwanzan", "mt. fuji", for example) that might be worth considering. the flowering crabapple malus florabunda might be worth looking at. since you want a good spring flowering show none of the japanese maples would be on the list despite their potentially excellent fall color. you might also look at some of the hawthorns (crataegus) might also be worth considering ---c. crus-galli "inermis" (thornless cockspur thorn), english hawthorn (c. laevigata "superba" and "red cloud") or washington thorn (c. phaenopyrum). all the hawthorns have bright red fall berries along with their colored leaves. remember most of the pacific northwest can have very dry summers so be prepared to mulch all the newly planted trees and water them regularly thru at least the first 2 summers. hope this helps.

You don't mention any size qualifications. If the fall colors are more important than flowers, I would not go with the cherries or crabs as they do not usually have very distinctive fall colors. The newer varieties of flowering pear are something you may want to look into. White flowers in the spring and red leaves in the fall. The dogwoods also would be good. Not just the Chinese but also the Eastern Dogwood. The redbuds also have good flowers and leaf color. You are in perfect maple country. The red maple and Sugar maple would give good colors. I do not know about the native Big Leaf maple. Vine maple also has good color. Avoid the Norway Maple and it's cultivars. A beautiful large tree would be the Tulip Tree aka Tulip Poplar etc. Sweet Gum is famous for the fall color. Pin oak, Red oak, Scarlet Oak all have great red colors in the fall. Nyssa Sylvatica is a great tree for fall colors. Living in the Northwest, you almost have too many too choose from. It's such a great gardening area.

Western WA is a great gardening area but it can present some challenges. Some trees are not good choices, primarily due to disease issues. If considering planting an entire 150' allee of the same trees, I would avoid any Prunus (flowering cherries, flowering plums) as they tend to be extremely disease prone and often very short-lived. Also the eastern dogwoods, unless you can locate one of the anthracnose resistant cultivars. And for whatever reason, redbuds here have issues........hard to find any that have any age to them at all.

Intended size is an important consideration. The flowering crabs are great smaller trees and would offer spring flowering but minimal fall color. Hybrid dogwoods can get larger than the kousas/Korean dogwoods with significant flowering potential but again with minimal fall color. Serviceberry is a good choice with both spring flower and great fall color but select for a tree-like form - many are more shrubby in habit.

Freeman or red maples are good choices. Avoid the native bigleaf maple - it grows naturally everywhere, has no fall color to speak of and can be prone to considerable breakage. Oaks are lovely but very slow. Katsura, Nyssa and Liquidambar are potentials for larger trees and all offer great fall color.