The weights used on your Tank gear is both a choice for you and situationally dependent – in lower damage situations you could play a more offensive setup and in harder content you want to be more defensive. It also depends on whether you are a Main or Off Tank.
7 Heavy is a great choice as this maximises your mitigation but also gives you extra resource gains from the Heavy Armor passives, and you are at the pinnacle of Tankyness with full Heavy.
There are some downsides to 7 Heavy such as slower movement speed and more expensive break free/roll dodge but these things are mostly outweighed by the Constitution passive. When dealing with really high damage situations, 7 Heavy will be the best option.
If you are using the Heavy Armor Skill Immovable you need to use 6 Heavy/1 Medium or Light to optimise the use of the Skill or else you are basically frozen on the spot for 6 seconds if you go 7 Heavy.
5/1/1 offers a range of additional cost reductions and extra recoveries but is not really any better than a 7 Heavy setup. There is a small difference in mitigation, its a few % less with 5/1/1 compared to 7 Heavy and this is a decent choice as you get a wide range of perks.
In the past you were forced to use 5 Heavy Armor minimum to benefit from the Heavy Armor Passives but this is not the case anymore and you can use as many or few pieces as you want of each weight and you will benefit from all passives per piece of each armor weight being used.
You do have to be aware of both Armor penalties and bonuses now as these also have various impacts on you as a Tank.
When Off Tanking, a 5 Medium 2 Heavy setup works perfectly fine especially since the Flames of Ambition patch where you are about the same strength in 5 Medium/2 Heavy as you were the patch previous in a 5/1/1 setup.
Light Armor tanking can be done but its not worth doing even in Magic damage fights, you still take a lot more damage than just using a Heavy Armor setup and there are also not good enough Light Armor sets to replace Yolnahkriin anyway.