Nothing breathes (new) life into a champion like a VO update.
There's lots of different parts of a full rework. There's kit changes, lore changes, visual, SFX, VFX, and probably more than I'm not aware enough of to name.
Personally though, the single most impactful - not necessarily the most important - is the voice over. Voice overs do a lot of things. They give a way to let the champion show off what they are, they give insights into the character themselves, their relationships, their ideals. Essentially, they flesh out the "character" part of the champion.
There's something deeply magical to me about voicework. It's a mixing of the efforts of multiple people and a testament to their individual skill to create a collective piece of work that shines. Dialogue writers have to collaborate with the lore writers to make the lines consistent, if there's additional SFX involved that's another layer of work. The dialogue itself has to work with the gameplay (Warwick's intense, often unintelligible growling is something I'd consider as an excellent example of this) and the voice actor has to be able to pull it off.
Not that the other aspects of champion creation arent equally impressive, I just have a soft spot for voice work.
And I think we've been extremely fortunate with the talent we've had in terms of voice actor quality lately...mostly. There's some outliers.
Anecdote time.
I've been playing a lot of Shen lately, so I'm going to use him as an example as I believe he's an example of how far just voice over work could go. I adore Shen. I have a lot of opinions on him as a gameplay entity and as a character but for now I'm going to focus on his VO. Shen got some nice visual and gameplay work a while back, but the VO side was neglected, leaving him with the same dialogue he's had since release.
It...sucks isnt the right word. For its age it's solid and holds up, but that's the highest praise I can give it, I can only compliment it in the context of how old it is. While Keith Silverstein's delivery is great, there's only so many times one can hear "With balanced steps" until it gets irritating more than immersive, or whatever hearing champion VO makes you feel.
Same situation with Vladimir but I'm not going down that road or I'll be here for far longer than I'd like to
Very recently, I played Maokai for the first time in about 15 months. This was the first time I'd played him since the VO and gameplay update, and I found myself really enjoying Maokai as a character more...or at least as much as you can. He's basically the same as before from where I stand but...more. There's more flesh to bite into, more dialogue and there's more said with that dialogue. It went a long way for me in regards to enjoying him, moreso than the kit changes honestly.
And an anecdote is just that, an anecdote. It doesnt really carry any weight or demand anything, I'm just sharing an experience that helps inform my views. I hope that through that I can inspire a similar feeling in others, or at least create an image that makes my stance make sense.
Something I would like to say is that I desperately hope Riot puts out some VO updates independent of a full rework, such as the one they did for Lux. Riot stated at one point they won't do any more updates that arent full VGUs (referencing the Karthus VU and how he probably should have gotten gameplay work too), which I think is a shame.
A VO update can be independent of a kit or visual update though, since a character can receive lore updates that give their character more depth to work with. I'm sure there's more than a few that come to mind with that idea.
It's something I hope Riot goes in the direction of. Waiting for full reworks for a VO update is...I feel there's a better way.