When it comes to gaming aspects and writing, it leaves a lot to be desired.
Hm, I think this is selling BG2 a bit too short.
When it comes to "gaming aspects", the game features some elements which belong to the best iterations in the genre, like itemization (which includes, for once, sensible crafting implementation), at times really good encounter design (Twisted Rune, anyone?) or complex spell system.
One other, at least from today´s perspective, notable characteristic is that despite its large amount of optional content and relatively open nature, the game is very light on enemy level scaling and there´s no item scaling.
While it totally doesn´t compare to the likes of Fallout or Arcanum, the game isn´t completely devoid of C&C - companions may leave based on your reputation, there´s a fairly substantial fork in the main quest, one quite big area that comes up during the main quest can be missed entirely, the Underdark area is quite strong in the multiple quest solutions department and the game features some class specific content as well.
The writing is neither of PS:T levels nor subpar, I think, and it does manage to deliver a varied portfolio of sometimes memorable characters, a memorable villain (though nonnegligible part of this comes from VA), sometimes quite elaborate side quests and one imo pretty cool and subtle twist in the main story (dream stuff).
And Athkatla as a computer game city is certainly nothing to frown at.
Anyway, the point is, I don´t think there are many party based games which have better "gaming aspects", writing, as well as the other qualities you mentioned (impressive amount of content, audiovisuals, UI)
at the same time.
In my opinion, THE selling point of BG2 is that it´s an unusually well rounded game.
It certainly doesn´t come with dips in quality as substantial as Ultima VII´s combat.
With the expansion installed, you could advance all the way to level 7.
I think it was 7-10, depending on class.
Never bought BG2 because of it, too.
Not a particularly good reason, considering there are, iirc, 28-40 levels available, depending on class.
I´d say more valid reason is simply character development rules on which the game operates on.
Unless you play one of the spellcasting classes, level ups don´t bring many "game-changers" to the table and, regardless of class type (sorcerer being an exception), levelling up offers very little in terms of character customization. However, in my opinion the game makes up for it with minute-to-minute customization options (spells, items), party composition opportunities and encounter design that is often good enough to "test" these.
As for the OT, I think every game on the list may serve well for scratching some particular itch(es) and I honestly don´t know what order I´d recommend to play these in.
Strictly positively speaking, Ultima VII is great as an adventure game set in an open, living and highly interactive world, Mass Effect is a great sci-fi popcorn entertainment, Morrowind is great as a lore infested hiking sim, DKS offers larger-than-life action/RPG experience and excellent use of humour, ToEE is a great DnD combat sim, Torment rulez with an iron fist at writing and being a truly "fantasy" game, Bloodlines delivers great atmosphere, Malkavian experience™ and characters, Baldur´s Gate Trilogy nets an epic, traditional adventure and an unusually well rounded set of features
.
All of these games also have good to great art direction and sets of flaws which shouldn´t be "gamebreaking" if you´d happen to enjoy the games for the above mentioned qualities.