[farmerjohn1324]

Something that will:

1. Protect from rain when necessary
2 . doesn't limit the height of what you want to carry
3. Possibly easily removable to meet #2

 

[ukrkoz]

Backflip. Folds flat against the rear window, so you can dump load in. Has options of opening in the back or in the front or partial opening. Locks. Pretty easy to install. Costs around $800 or so. I liked mine a lot, it was VERY functional.
Leaks eventually. All folding ones do.

 

[lenaitch]

I have a tri-fold that I like; not a true back-flip since the section closest to the cab is fixed. It's not that hard to remove completely but heavy. Not all are completely weather-proof. There are online comments that my make leaks - I haven't found that and I figure some extra weatherstripping would probably solve it if it did.
Some folks like vinyl roll-ups (true 'tonneau cover'). They are a lot cheaper but lower quality material can make dome fastener re-attachment difficult after a while. I'm not sure how well they do with snow load but, given your other posts with outdoor plumbing I'm guessing that's not an issue.
I have had caps in the past but found them a pain to remove-re-install if you need more height.
I don't really worry about security as it is not an issue for me and, besides, my truck doesn't have a locking tailgate.

Lots of options and lots of variations within different options.

 

[DexterII]

Just like the vehicle itself, everyone's going to have different likes and dislikes based on individual experience and needs, but my preference is the Truxedo LoPro. Had one on my last truck, for about 10 years, bought a trifold for the new truck based on what I heard some other guys say about them, and have regretted it since about the first week. Granted, 90% of the time it's fine, it seals good, doesn't leak, and is easy enough to remove when necessary. But the Truxedo rolled up right to the back of the cab and I never had to remove it even once in 10 years. The only bad thing was that some of the stitching was letting go the last couple of months, allowing the sides to flap a bit sometimes, but, again, it was 10 years old so in hindsight was not too disappointing.

 

[3onthetree]

Topper/cap?
or Tonneau/cover?

 

[Skookum]

Topper/cap?
or Tonneau/cover?

I will say, if you want something that will increase space for storage go for a topper. It does limit your height and the ability to remove it can be a royal PITA if you are doing it solo (ask me how I know). LEER or ARE are two well known brands -- if you can afford it


I like the Tonneau's. They are slick, and doesn't add too much weight. But I don't know of any that allow you to lock with a key.



I've seen some with little hydraulic parts which open the whole thing up on the backend -- I cant recall the brand on it. The good thing is that the hydraulics also keep it from opening up and keep it closed.

 

[Colbyt]

All of them are removable with a helper or a hoist.


I bought a fiberglass like the top image above in 1996 and it is still going strong and so is the truck.


If buying new today I might consider one of the tri folds.

 

[Old Thomas]

I have owned a few trucks with Extang roll up tonneau covers and they meet my needs well. I like to have a cover that is easy to get out of the way without help and when I am away and didn’t plan to need it gone. It is easy to release and roll it up if needed to a tight roll behind the cab. They have bows that roll up with the cover and have had a couple of feet of snow on them without any problem. Not a drop of water inside after driving in pouring rain. My last truck had one for 10 years and 100k miles and finally the stitching came loose at the tailgate end. The cover had a lifetime warranty so I emailed them. After I sent a pic showing that I had it and which model, they sent me a brand new one free. Because of that, I bought one for my current truck right after I bought it.
I was on the interstate a few years ago north of Pittsburgh and a pickup entered the highway in front of me. Just as she hit 70 mph her trifold cover separated from her truck. It flew in the air and I almost missed it. Almost missed it-$3000 damage to the Mustang convertible. Glad it didn’t come through my windshield.