I have to admit, I think 'RPG' is the most abused term in computer gaming. So many games companies don't understand that term, so they wrongly label their games as RPGs because they think anything where you level a character up or equip them is an RPG. Which it's not, of course, but unfortunately the players of RPGs never bothered to protest when the problem started (And yes, I'm as culpable as any other RPG player for this) and now we're stuck with a definition that could be applied to almost any game you care to name. It's an utterly ridiculous situation.
True RPGs are determined by one thing and one thing alone: characterisation. Role playing is freeform acting; it is assuming a role. As such, the player must be able to affect the personality and characterisation of the character they play.
Even with set characters (As in The Witcher) there can be a huge amount of personality variance if the game is done right, because of the choices you make as the character. In a true RPG.
So yes, you are right. Diablo 2 is no kind of RPG at all. Most of the games on that list aren't (And I'm only saying 'most' because I haven't played some of them so can't be sure). But, unfortunately, far too many people think they are for that battle to ever be won.
As for which of those games is the best, well, that's always gonna be a matter of opinion
I fully agree. The "matter of opinion" part was why I replied to "Diablo 2 is probably the best ARPG ever made.", though - you made it sound like that wasn't a matter of opinion.
To be honest, there IS some justification for calling Diablo 2 a role-playing game. It wasn't the first game that centered on fighting and leveling up while still being labeled a role-playing game. In the contrary, many of the early (and very successful) computer role-playing games are about exploring dungeons, fighting and leveling up - and little else. Think of the (original) Bard's Tale series, the Dungeon Master series or the Eye of the Beholder series ... so much for "old school" and sequels being a recent development, by the way.
And the tradition dates even further back. Many adventures for the D&D and AD&D pen & paper role-playing game centered on combat - you had to fight numerous monsters and gained far more XP through fighting than as a quest reward. Those adventures were called "hack and slash", but they were still thought of as being part of a role-playing game.
In this sense, Diablo 2 is a role-playing game. I do think that the standards for computer role-playing games have changed, however, and that the same term should not be used for Planescape: Torment (published in 1999) and Diablo 2 (published in 2000). As it follows the tradition of adventures purely centered on combat, it's fair enough to call Diablo 2 a "hack and slash" game, whether you want to see this as a sub-genre of role-playing games or not.
I think the "action" in "action role-playing game" originally pointed out that the game had real-time combat, not turn-based combat. Since there are so few new games with turn-based combat, "action" has come to mean "combat-heavy", and of course, "action role-playing game" does sound better than "hack and slash game", so publishers prefer to use that term.
