For those that have been to these areas, please rank the order of importance to visit these areas and why you chose your order:

Redwood National Park area
Sequoia National Park area
Yosemite National Park area

Of the 42 US national parks I've seen, I'd rank them as follows:

Yosemite (actually, one of my top three)
Sequoia and Kings Canyon
Redwoods

Note that you can see impressive sequoias in Yosemite, and coastal redwoods in several state near San Francisco. But there's only place in the world you can see Yosemite Valley.

#1 Yosemite w/o question -- Especially IF you can stay in Yosemite Valley

#2 RNP -- IF you have enough time to also explore Ferndale, Ft Bragg/Mendocino and/or the Oregon Coast. But if you just want to rush up 101, take a peek and rush back -- too much driving for too little time

#3 Sequoia/Kings Canyon. Though it moves up the list IF you don't have time for the north coast and can combine Sequoia w/ YNP.

There is really no way to answer w/o more context though. How much time do you have total? What time of year? Where are you starting from/heading to? Can't answer in a vacuum.

I would definately rank Yosemite as #1. I think I am up to 25 Parks and I rank it #1. My wife and daughter rank it #2 behind Yellowstone. To me, the most impressive trees are in Sequoia and least impressive are in Yosemite. The Redwoods are just different. And really the drive through the Redwood Trees is the best. It seems they go on for miles and miles, while the Sequoia's in Yosemite and Sequoia are just in groves for a few acres here and there. The groves in Sequoia and bigger than Yosemite's Groves. The diameter of the trees in Sequoia are just so massive. All three parks the trees are very very impressive. The Waterfalls and view are what Yosemite is really about for most folks. I do think that The Redwoods have some amazing hikes. Yosemite can get really crowded, so if you like to be on the trails with little and even sometime noone, you might like Redwoods or Sequoia best. I would rank Sequoia even with The Redwoods, it just depends on what you want to see. The Redwoods has ocean views as well. Fern Canyon is a pretty spectacular easy hike. But then again, seeing the largest living thing in the world(General Sherman Tree) in Sequoia is pretty darn cool as well. I can't imagine any of the three not leaving an impression on you. I agree with janisj, give each park as much time as you can. It might make a difference which month you are planning this trip as well.

Yosemite
Redwoods
Sequoia

in that order. And I agree with spirobulldog's wife and daughter on Yellowstone. I've been to Yosemite at least 30 times, because it's practically in my backyard (well, in my state, anyway) and to Yellowstone only once, so I may have been overly awestruck by the geysers and hot springs, especially the color of the latter.

Yosemite #1 hands down. Beware thick crowds in summer. Make reservations early.

Haven't been to Redwoods.

1. Yosemite.

2. Kings Canyon.

3. Sequoia.

Another vote for Yosemite as #1. As I said in another post the Giant Sequoias in Yosemite and Sequoia are a different species from the Coast Redwoods in Redwood NP.
Redwood NP is easier to add to a visit to the southern Oregon coast and Crater Lake National Park than San Francisco, Yosemite and Sequoia.

I agree with Paul Rabe.

Thanks folks. That helped cement it for me.

Now my biggest effort is determining how much time to leave for the trip up the coast from LA to SF. I can estimate the travel time and time touring a specific place (such as Hearst Castle, Mystery House, Monterey Bay Aquarium, etc), but how much time to allow for walks, hikes and viewing the scenery and experiencing something that I am not even aware of now is tough. Right now I am allowing 6 days for the trip from LA to SF including all the tours, scenery watching, etc.

Sounds about right. What about Channel Islands? Are you going to do it? It is high on my list and something I hope to do someday. To do it right would require camping on an island for an overnight or two. I imagine that a daytrip to an island would be a good experience though. My wife and I are doing a long weekedn trip in June just driving from San Fran to Hearst Castle and then back. No Channel Islands on this trip.