So with the blessings of being in the position I am in, one of the things I get to experience is the feel of different materials for bicycles. Not just any ol' bicycles either, but really well executed ones. One thing to keep in mind, a frames material is only as good as the engineering and design behind it.
Steel: I suppose just about everybody has ridden a steel bicycle frame at one point or another, and you can debate until the cows come home about whether or not "steel is real" or even what that means. I will say only that a great quality steel used well, like the 853 Reynolds in this Raleigh XXIX, feels pretty darn nice used in a hard tail application. That said, even steel without a "pedigree" can feel pretty nice if the design is correct. Witness the OS Bikes Blackbuck, or the Salsa Fargo for example. Steel also has that advantage of being better at not failing catastrophically. (Not that it can't) And it can last a lifetime with some very reasonable care and maintenance.
Carbon Fiber: This is the mystery material that has people skittish or all hot and bothered. Again, it takes a great design and great execution of that design to make a great carbon frame. I've had the privilege of piloting a Specialized Stumpjumper Carbon Expert lately and I can say it rides pretty sweetly. It isn't without some faults, (A bit torsionally flexy), but it does a stellar job of smoothing the trail in an odd sort of way that is hard to describe. The big rap against carbon is small failures and catastrophic failure fears. I have been hearing about a lot of "cracked" carbon frames, but cracks that are caught out are much better than having a frame disintegrate underneath you on a ride. Thank goodness I have not experienced either cracks or failures in my carbon fiber experiences. That said, any failure, crack or catastrophic, of any bicycle frame is a bummer, bad, or at worst, injury causing. It isn't a "carbon fiber thing" exclusively. My experience recently riding the rocky trails of El Paso, Texas show that carbon can be pretty resilient. Rocks were pinging off the frame like crazy! However; the high price of entry for a carbon fiber frame, like the pictured rendering of the upcoming Orbea Alma version 2, makes even the slightest damage a very great concern.
Titanium: This isn't a very inexpensive alternative, but in a way, it has some of the features of both carbon fiber and steel, all wrapped up in a pretty durable package.
The titanium story is one of compliance and trying to reign that in so that the ride isn't too flexy. A tough cookie to crack in a 29"er size. Of course, you could throw a bunch of material at the design, but that defeats the allure of titaniums lighter weight advantage. So, titanium hasn't really been all that successful as far as making a great 29"er until recently. As far as durability, titanium really doesn't have a corrosion issue, it doesn't need paint, and can look as nice in 20 years as it does now. You don't have much to worry about as far as failures go either, as long as the frame was prepped and welded properly. If you have the cash, it's hard not to really like titanium.
Conclusions: As far as "value" goes, nothing can touch a steel frame for a bicycle. It can be reasonably light weight, tough, ride nicely, and last a long time for little cash outlay. Is steel "real", or is it really "king"? Tough to argue against that. Carbon fiber is certainly king of light weight while retaining strength. It can be "tuned", but at a greater expense than even titanium in some cases. Due to varying supply originations, it is hard to say what is good and what isn't at the off-brand, lower price scale with carbon frames. In the end, the jury is still out on durability too. Last we have titanium, which if the latest designs are any indication, is making some solid inroads as a great material for a 29"er. The material is certainly durable, rides sweetly, and can retain some of that lighter weight advantage against steel. It is still a pretty pricey proposition though, and there are not too many really outstanding choices in titanium that are readily available that don't have the name "Lynskey" attached to them somehow. (Not that this is a bad thing, I'm just saying...)
So, I would still hand the crown of "best material for a frame" to steel. It is just hard not to like a material that is so well known, so available, so full of potential for performance, and is /can be a real inexpensive choice. That said, carbon fiber is sexy and titanium is dreamy. There really isn't a "bad" choice out there from well known, established companies. In the end though, you'd likely see me happily dusting it up on some steel frame and never looking back.............well............unless that design came in titanium...................or if that carbon beauty wasn't quite soooo spendy...............or..................