This is my personal view - considering that the word "threat" is flexible. It could mean something that is very much used as like, - a "utility counter", hardcore wall , or defensive pivot, - but not really something that puts the fear of God into a team, such as Pert, Skarm, Bliss, Dol, Clops, etc or even something like P2 or Saur. I would not call the aforementioned pokes "threats" - I would call them "counters to threats". But I've taken the word "threat" to mean something that actively offensively threatens the opposing team - can weaken counters, etc. This does not necessarily have to be a sweeper.
I chose the threats for varying reasons - some because of their versatility and unpredictability, such as Tyranitar - these days when switching your Pert into TTar you'll always scout for HP Grass with Protect/a switch as opposed to EQ'ing/Surfing/HPumping. TTar is what the meta revolves around - it really is. Personally I hardly ever use it, but hey, that's me.
I chose some things such as Dugtrio for simply being so good at what they do. While not being as versatile as the likes of TTar and Celebi, they are still extremely effective. You know what's coming when you see a Dugtrio and there's nothing you can do about it. To pick out a non-trapping example, there's Gyarados. You know what's coming when you see it - Taunt, Dragon Dance, HP Flying , EQ, occasionally Double-Edge over Taunt. You know you can fairly safely switch Zapdos into standard Gyara and come out the other end relatively unscathed, but Gyarados, with correct support, can plough through whole teams with correct support.
I nominated some things such as Suicune, Zapdos, Jirachi, because as well as being offensive powerhouses, they have decent bulk and can switch in. Take Suicune for example (the poke I nomianted at number 3) - while Suicune is an infamous wall, it also doubles up as a threatening Calm Mind sweeper that can stall some of its would-be counters to a boring death with Pressure. However, this is not to say that Suicune does not have its issues - it is damaged by the abundant sand and worn down by Spikes. In the right environment though - i.e. no weather, not much prior damage, all forms of CM Suicune, especially the underrated SubCM one are devastatingly powerful.
That's all I feel I need to say about my selection process - it's very difficult to rank pokes in ADV in a strict order - you are often comparing two completely different things with totally different roles.