From what you described, I think the best single-chip solution would be the flexNT. The flexNT and the xNT use the exact same chip (an NFC NTAG 216). The flex has significantly greater read range. The flexNT will give you the maximum range and performance
The advantage of the NTAG 216 chips is that there are quite a few existing devices that work with them. And the chips are semi-programmable so you (or others) can also write client applications that could make use of them. They also work with any smartphone that has an NFC reader. You can easily store a vCard or many other types of NDEF data on the NTAG216, and on most Android phones, just tapping the phone to the chip will bring up a dialog asking to add the contact, send an email, open a webpage, etc. A couple of caveats though. First, having multiple NDEF records is a mixed bag. Some phones will only respond to the first. Others (like my Samsung S8+) will respond to multiple. So if I have a vCard NDEF record and a webpage record, both may open. But my friends Huawei sometimes opens the vCard record and sometimes does nothing. Second, there is only limited support on iPhones. Search the forums for more info, but only the last few generations of iPhones started having even half-way decent support for NFC.
Commercial Building Access: As others have said, this depends on the type of ACS (access control system) your building uses. If this is the pad you are talking about, then you will not be able to use the flexNT/xNT to access it. Only the xEM would work for this, and even then I give it a 50/50 shot at best. In order to get any new RFID chip to work with an ACS, you have two strategies. First, if you have access to the ACS system, or can convince the building owner, it is usually pretty simple to register a new chip with the ACS. You or someone else would “enroll” the xEM. If this isn’t possible then you might be able to clone your existing RFID key to an xEM. But this requires purchasing or having access to a device called a ProxMark, along with some knowledge of terminal commands. It isn’t difficult, but it’s not a one-click operation either.
For Phone/Computer/Account security, the NT is again your best bet, at least for now. The simplest way is to use your flexNT to augment an existing password. You would type in the first part of your password, then use a USB reader to scan your flexNT and have it spit out the UID. This UID would form the second half of your password which would then let you log-in to the computer. Dangerous Things sells one of these readers. The computer sees it as a keyboard, meaning it can be used anywhere you would enter text, so it can augment any kind of password. I would not recommend using it as the full password, as there is nothing you can do to prevent anyone with an RFID scanner from reading the UID off your tag. The fact that it is implanted in your hand helps, but it is still pretty easy to get the UID if someone is determined. You can also program other kinds of data on the flexNT, and the chip does have some security features for protecting that data from both reads and writes. However it is complicated to setup, and if you wanted to store your own “secrets” you would need to code your own computer application to read it.
The forthcoming VivoKey Flex One is both more and less functional than the xNT. It if more functional in that it is fully programmable and can, in theory, do a ton of stuff. However, it currently is less functional out of the box. There are no access control systems, door locks, or even phones that can interact with it natively. The Flex One does have some cool uses, and one of those is as a replacement to Google Authenticator. This is already working and opens some cool account security uses. But this is more for protecting online accounts like Gmail, Facebook, etc, then for logging into your computer locally.
Bottom line, if you really, really only want a single chip, ever, then wait for the Flex One. But it could be a long time before it does everything you want it to. Instead, I’d get the flexNT now, make use of the low-configuration uses like vCards and unlocking your door, then getting a Flex One in the future when it’s ready.
If you are willing to go under the knife to get a flex, then getting an injectable xEM to cover other scenarios is an easy solution.