I suppose this is probably something that gets brought up a lot, however through a bit of research I never really came to a conclusion and I would prefer to make an overly-large and inconsistent-with-the-subject thread on it rather than keep looking for an answer I doubt I'll find without help.

I guess I should open by saying that for the most part I do usually have a shield for most situations, so this is not really a recommendation thread, moreso a discussion thread. That said I'm making this for the sake of recommendations.

What I'm looking for mostly is simply a shield that is completely all-purpose and can be used as a shield where you go in saying "I don't know what to pick so I'm going with this".

From that "criteria", one of the first things that comes to mind personally is Grey Owlite Shield (and the reasons should be obvious enough for this one), or Crest of Almire in its place, although I personally feel that Grey Owlite Shield is the better choice considering that Gun Puppies are more common than Howlitzers, and Mecha Knights tend to be a major threat to begin with considering lag.
But parting from that, I was looking more specifically in to Ancient Plate Shield vs. Omega Shell; the general consensus seems to be that Ancient Plate Shield is superior because of its heightened Normal defense (although oddly the fact that Ancient Plate Shield has stun resistance seems to be left out). That brought me to Ancient Plate Shield versus Grand Tortoise (Grand Tortoise being the shield I now almost always use as an all-around shield), but some simple information seems to have already debunked Grand Tortoise's usefulness, although I do shield bash with Grand Tortoise almost all the time, usually as soon as my bar reaches max from my last shield bash, so I'm still somewhat biased towards it.

While there are other shields that provide their niches and could be mentioned, such as Dread Skelly Shield for its beautiful defenses similar to Grey Owlite Shield, and Volcanic Plate Shield for its similarly good tanking ability to Ancient Plate Shield but with Fire resistance on top, the last "all-purpose" shield I could think of is Swiftstrike Buckler. Obviously this is the go-to shield for many-a people -- myself included in most cases, but I was also going in to this assuming that the enemies I'd face in my metaphorical depth where I metaphorically pick an all-purpose shield would be a little stronger than most dull-minded mobs you'd find floating around in FSC or a Wolver den, so wearing Swiftstrike Buckler would (preferably) be suicide.

On that note, Barbarous Thorn Shield is another obvious option; being in the mindset of "I don't know what to use so I might as well get a bonus for my weapons" is always a choice, and again I'm just assuming Swiftstrike Buckler is out, but Barbarous Thorn Shield has defense to compensate.

So really as an all-purpose shield, I've been wondering which to go with; Grey Owlite Shield for its immaculate defenses, Omega Shell for its vigor, Ancient Plate Shield for its monstrous Normal defense and resistance to getting swept by Lumbers and Trojans, Grand Tortoise for its shield bashing abilities, Swiftstrike Buckler just because ASI is invaluable, or some other shield that I just didn't think of off the top of my head? Barbarous Thorn Shield maybe? Something else?

Perhaps I lost track of where I was going with this thread -- it will probably see its grave soon enough.
TL;DR- just, what would be an efficient all-around shield to choose when going in to a depth blind and not knowing what to pick otherwise. Of course, this depends on a lot of other factors, but just assume you're going in completely blind with no knowledge of what enemies the depth holds or even what status(es) it has.
---- Sorry if my writing is too sporadic, it's pretty late....

EDIT: Fixed a broken link and added some information..