Tahm Kench, Gangplank, and Braum are some of my favorites. Least favorite is Yasuo because I'm tired of hearing "Hasaki" every 2 seconds.
is my favorite, His voice fits really well with his personality. Helps that I like playing him and like his lore too.
is the worst. His voice doesn't fit his look nor his personality.
Favorite :

Least Favorite :

I don't even know... 
I really love Champion voices that sound other-worldly. Guys like Karthus, Aatrox, Tahm Kench, and Aurelion Sol come to mind immediately.
Least favorite are voices that just sound completely forced and seem like parodies more than a serious character. Warwick, Swain, and Mundo honestly sound really bad and I believe were done in the earliest days of League, so Riot's budget and the standard they set for themselves was probably extremely low.
I tend to really like the calm and echoey voices like Nasus, Skarner, and Kindred.
My least favorite champion voice is, by a large mile, Annie. The delivery on her lines makes me cringe, for some reason.
Braum, Tahm, Thresh, & Zac are often my favorites to listen to in game; so much amusing personality from their lore.
Gnar, Fizz, & Teemo are like cheese graters. I know it's the voice modulation, but I'd rather hear urgot's wheezing than those dog toys.
is amazing, that video sucks.
Favourite is
, it makes her character even refined and graceful....when she isn't shouting randomly that is.
Least favourite is definitely
because I'd like to hunt this person down and punch them in the face(or better yet punch Zed in the face).
While not a VOICE actor, did an amazing job with this "voice."
A lot of the champions have really good voices that coinside with their lore ( Kayle is all serious. Tahm Kench is playful in a sense. ASol is an arrogant prick. )
The voices are great, the older ones just need some tuning.
Fav = Pantheon Hated = Sona
guess i don't have any particular favorites but my least favorite are tristana and lux
Kindred's voice is pretty majestic, but I'm pretty dissapointed in Rammus'
Ryze currently has the worst voice out of any champion that isn't Rammus (because he doesn't really have enough actual VO to count).
Which is funny because Ryze's VA does a few other champions too, none of whom sound anywhere near as awful as Ryze does.
Fortunately, Ryze's upcoming rework includes a new VO as well, as far as I'm aware.
sounds like he didn't fully go through puberty
Taric, Kench, Illaoi, and Taliyah are absolutely fantastic. Rammus is awful. Yi is an affront to Asian Americans. Ryze is just terrible, but he doesn't count anymore considering his rework coming.
has like 4 lines of dialogue in total... And they're all short and uncreative