Jagged Alliance 2 stands above all other games in the RPG genre, in terms of doing well in pretty much all categories, although it's by no means perfect, of course.
However, while it's clearly a game with heavy RPG elements, it's also much more focused on the turn based tactics side, so proclaiming it as the best RPG ever (which I'm going to go ahead and do), is a statement that's destined to be controversial as it implies certain things about the RPG genre.
Combat/Gameplay: A+, stands the test of time to the point where people have played it regularly for a decade, often using the same maps for most of that. Not much comes close.
Characterization and Dialog: A+, these are almost certainly some of the best characterized NPCs in any game, and the quality of dialog is similarly amazing
Choices and consequences: There are quite a few, and they emerge through gameplay, rather than simple tactics ogre style "do you burn the village or not" menus. Most of the choices and consequences however emerge from the relations between NPCs (Steriod hates Russians, many people hate Buns, etc) and in gameplay rather than from options in the plot to turn evil or whatever. On the other hand, it's nice that you can hire both people who hate each other, as long as you try to keep them in separate squads and only rarely bring them into the same sector they won't automatically leave.
Plot: This could be considered a weakness, as the plot while fun and well done, is not that much of a driving force, as it's more of a sandbox game, where you're left on your own to accomplish a mission and the story is told through interludes and dialog with npcs than a strict chapter progression. But it's not like it actually would get a bad grade here either.
Graphics and sound: technically limited but extremely functional and charming; these have stood the test of time as well.
Vampire, fallout and arcanum, for all their major strong points, still suffer from bad-mediocre combat. BG2 for all it's many strong points is still much more flawed than JA2, in terms of combat and arguably in other ways as well.
TOEE and otther hack and slashers can at best hope to come within 85% of JA2's combat and entirely lack the other qualities that make it such a timeless classic.
Really, I'd have trouble seeing anything that comes close to JA2 in terms of RPGs which are overall great and lack that annoying flaw that most of the other top dogs in the genre seem burdened with.