After 9 years, I still find sorcerer to be my favourite class. Neither introduction of Warden, Necromancer or Arcanist, nor the coming and going of the BiS metas/"viability" have changed that for me.
A couple of years ago, one of the developers mentioned that Sorcerer and Nightblade were the most popular classes. Although various mechanical reasons and ease of play/accessibility are considered significant reasons for that (particularly for sorcerer), I think that a possibly overlooked reason is that these two classes capture the feel of classic Elder Scrolls character archetypes (at least, conceptually); if a large number of the player base are recruited from the classic Elder Scrolls games, it stands to reason they might gravitate to classes that would feel like Elder Scrolls characters.
Sorcerer gives me the best possibility of playing a classic mage. I absolutely love the iconic Hardened Ward in both gameplay and visuals- it feels like the mage's answer to damage mitigation and is drawn straight from Morrowind's "xth Barrier" spells. I hope they never change the visuals for it.
Twilight cops a lot of flack (its a hotbutton topic for the forum-goers), but I've always loved it. Its a core identity spell for sorcerers, and I'd hate to lose it. Its another possibility that they drew from Morrowind, and I love having it. I empathise with people who find the fluttering annoying, but I hope that ZOS remembers that although you don't see it where all the raging and fuming happens on the forums, many playing the game still treasure it.
I've said it before, elsewhere; the image above illustrates the Twilight's method of getting around in Morrowind- the game in which it first made an appearance. I do think it would be a good compromise to have them earthbound like this, taking only to wing when in combat or when their controller is sprinting. The new models of "walking" Twilights in Apocrypha might suggest the developers are thinking this way, too.
Honourable mentions for Mage's Wrath (which is a powerful Daggerfall spell, and perfect as an execute), scamp (though he should not be purple!), and curse/frags/mines (because the gameplay niche was fun and iconic).
The only thing I've never liked was a line dedicated to an energy type (Lightning), because it restricts them too much. Mages in the older games were masters of *all* elements, or specialist in *any*. Trying to make a build that ignores the prime importance of lightning to the class's toolkit gimps you unnecessarily.
I'm glad to hear that the Arcanist was a huge success, and happy that a lot of people are having a lot of fun. Its natural that people have begun to ask about the older classes, going forward. I just hope that if that's done its not done in a way that would obliterate the things some of us really cherish.