For whatever it's worth, among former types I've flying I've been paid to do, has been dropping on fires, spraying on crops, flying mechanics, freight, and equipment, remote area air ambulance, back country work, scenic tours, fire patrol, sheep counting and animal tracking, search and rescue, jump operations, flight instructing, corporate flying, aerial photography, weather modification and treatment, banner towing, ferry and repositon work, time critical ALS air ambulance, etc.
Most all of that experience is pooh-pooh'd by the grand poohbahs at many companies. The equipment flown has ranged from light fabric piper airplanes to heavy radial and turbine powered four engine equipment. You might think that large four engine airplanes operated at gross weights under extreme conditions at low level in tight quarters and proximity to terrain and other aircraft might suggest some good operating experience, but I've been told by more than a few folks in the past that it's only piston experience, and doesn't count for much. Go figure.
What's the most valueable? Whatever gives you the most experience to craft you a better pilot, and gives you judgement. Otherwise, make the rest up, write it in yourlogbook, and be done with it. The best job is one that pays you, one that you needn't pay for (does anybody remember having to pay to rent an airplane as a private pilot, and how exciting the first paid trip was when someone else was footing the bill??).
I can't look back and say that any one kind of experience has been more valueable to me than any other. I hold all the memories dear, and I'd perform any one of those jobs again in a heartbeat...or any of the other jobs I've done.
The only truly applicable experience to your next job is the job itself. However, how the former work applies is very much how you see it. A pilot who has been flying a F-16 doesn't arrive at an interview for a B737 job and say, "I don't suppose you need someone to fly extreme aerobatic formation work in this airplane, do you, because that's my strong suit. Hey, where is the fire control computer in this thing? What, no threat warning? I'm out of here." Won't happen.
Instead, that pilot will present himself has having good solid training in the fundamentals of aircraft operation. He will show that he has a proven track record operating high performance turbine equipment, strong instrument skills, and the ability to make critical decisions on short notice and make them correctly with mature judgement.
Likewise, the pilot who has been flying banners, instructing, or freight, doesn't arrive at the interview to ask, "So, like, where does the tow ring go on this thing? Do I get to sit right seat, or left? How many chickens can I cram in that sucker (can I say that in mixed company)? Do I still have to fly over gross?"
Instead, he or she might show that judgement has been gained flying in mixed and demanding conditions, lower altitudes, in the clouds. Lots of instrument approaches. Schedules have been met with the freight. Good crew experience working with students in the cockpit; really helped to be able to talk, think, and fly at the same time. Still likes to stay current. Flew a variety of equipment, took regular checkrides and proficiency exams. Whatever.
How the pilot sees his or her background and conveys it is more important than the background itself. How you've conducted yourself during those years in that background makes a very big difference. Did you act professionally, or treat it like the "fun years" until you had that real job you've been climbing the great curtain to get? It's hard to represent as valueable experience time spent not acting like a professional; you'll know it, and it will show. What's in your wallet, so to speak? (I hate those commercials).
The most valueable experience is what you make it out to be. Sell yourself. Sure, you would be best off with a type rating in the aircraft for which you interview, and 5,000 hours or more in type doing the same job for the same company, with an outstanding service record. If that isn't you, then take the job you can get, do it as professionally as you can, and let your learning and your expeirence, and your character speak for you. Thousands upon thousands of others in the industry have done just this; why not you?