If you can only do one, the Cascade loop. Rt 20 to Rt 97 and then back on Rt 2. Go into Wenatchee for food and gas. Not sure when you are planning on going, but Rt. 20 can be closed this time of year due to snow. They do not plow the middle section in the winter. With all the snow we have had in the west this year, it could be June before they get it open. Check before you go unless it is in the middle of summer.
Rainier is beautiful. The North Cascade Loop would be my choice, with a nice day trip into Leavenworth. The loop around the Olympic Peninsula with a day in the Olympic National Park would be a great ride as well. Plenty of little crab shacks and ocean views to be had, and the scenery in the Olympic rain forest is like nothing you'll see anywhere.
For the best scenic ride Mt Rainier would be my pick. The ride up to Paradise is a must see. If you decide to make the trip Packwood has a place voted best hamburger in Washington state. Cliff droppers on highway 12. 5 miles away at Ohanapecosh national park. There is a fantastic hike to silver falls to give ones buns a well deserved 3 mike break.