Sorry for the long post, I may have went slightly overboard
Pingtest.net is good only if.you're testing pings against a particular location under non-load conditions, otherwise it just chooses the closest server and will always have a good result, unless you have worse issues somewhere else.
In any case, your ISP can't be blamed for giving you those speeds if it truly says "up to # Mbps", as much as I'd like to say otherwise. You reach 16; that's still under the "up to 50" statement, and they will face you this motivation if you call them and they find nothing, after checking your network's conditions of course (they can't refuse checking if you ask).
However, if you're getting less than the minimum guaranteed bandwidth, provided your contract states it, you have all the rights to complain, so be sure to check that out.
Other than that, what titillating said about required bandwidth is true. A good minimum value (with some headroom for good measure) for smooth internet play, provided you're connecting to a dedicated server and that there's no load on your home network, would be D 5/U 0.5 Mbps.
As per the issue you're experiencing, a little explanation of my opinion on it. Skip this and the next paragraph if you want the short answer and don't care about all this
The issue for you comes when your download bandwidth is saturated by youtube's traffic. Every time you move in an online game, you're communicating your new position to the server; that usually gets sent relatively fast, as your upload bandwidth is almost constantly free.
At the same time however, you have to download the position of everyone in your same location/lobby/playing zone/map/whatever at regular intervals, which includes your position as well.
Let's call each of these intervals a snapshot. Snapshots' data ALWAYS have priority over the last sent data, to ensure everyone out there in the world is working exactly with the same set of information.
If a snapshot says you're at point A, but your current location is B, you get teleported back to A and start moving afterwards, while your current point B gets ignored and trashed.
To avoid this, you have 3 options:
1 - exclude anyone but you from accessing the internet. Easiest, fastest, but obviously rude and mostly too drastic and unwanted, like you mentioned already...
2 - get a higher bandwidth in proportion of the number of devices used contemporarily. Those 16 are ok already for 3-4 devices streaming at full speed maybe. I used to have 6, and I struggled with 2 computers, got 20 now and I only start struggling at 5 or more devices.
3 - limit the bandwidth of all devices but yours to a certain value by router settings. Not all of them allow to do so, but if you find the right settings, this would be a more than viable option.
If your router allows device registration/labeling and has per-device configuration, it may allow you to do so.
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