Hello champions, good to see you.
Okay, before you start typing "WHAT ARE YOU SAYING LOTUS? ARE YOU INSANE?", listen to me, to why I think that GW2 has the best progression system in any MMO so far.
I played Ragnarök, Perfect World, World of Wacraft, Rift, Runescape, Lineage 2, Tree of Savior, Mu Online, and many others. Most of them offer good progression systems, Ragnarök being my one of my favourites as well.
But I think that Guild Wars 2's progression tops all of those MMO's - GW2's progression is great: You level up with everything, and most of the content is easily accessible, even at early levels. It may be more casual, yes, but it still fun as hell.
Yet, people complain that GW2 has no end game. In the early days, I kinda agreed with that statement. Sometimes you feel lost, because technically you don't have anything to "look out for", to "achieve".
This brings me to the main topic:
Heart of Thorns (and, in the future, Path of Fire) brought Masteries to the table. And I think that Masteries are one of the most incredible things that any MMO has ever created - You unlock things that your character can do, but you don't reset your old progress with that. This brings imaginative ways to progress your PC, and you don't just discard everything that you did on the last year or so while doing that.
Let me give an example:
In MMO-X, you level up to level 70. You farm equipments, get the best loot, etc. Then, after a year and a half, a new xpac comes, raising the level to 80. All your old equipment, all your old progress...is thrown away. You don't keep anything, and everything offered by the new xpac is way better than the gear that you worked a lot in the last one.
In GW2, you keep your equipment, so no need to keep farming new equips - you're already ready for the jungle, you're the Pact Commander, after all. Then, you learn new ways to navigate the jungle - Gliding, jumping on mushrooms, learning new languages, etc.
For me (Read that again, until you understand that this is MY opinion), GW2 nailed the progression with Heart of Thorns - We could use a little less grind on it, yes, it's not a flawless system, but it's what MMO's should be doing nowadays - Your character should evolve, not your equipment. You already have an epic sword made with magic dragon bones, so why would a "green" plastic knife be better than it?
TL/DR: Heart of Thorns nailed the progression system in GW2, and Path of Fire is looking to do more things with it. I think that the progression system in GW2 is better than other MMO's because of that.
PS: Sorry for my english.