Seriously, I can never get one. Just S's, S-'s, A's and A+'s. How exactly is this grade determined? I try to participate in as much as possible, objectives, kills, warding and even gold leads(this includes CS, which I try not to miss too much of) but I can never get that +. T-T what am I doing wrong?
Raka Flaka MD4 votesPlay nami jungle and go 10-0-10.
Catastrop1 votesI don't own her ;-; Although she's been on my Buy List for like a year and a half.
Raka Flaka MD4 votesFor real though iv had games where i go 6-0-20 with 400 cs and not get an S+ and had other games where i got carried and ended around 4-3-10 and gotten an S+. It's based on the champ your playing and the role its in. So if you play say, Nami jungle and do good you will get an S+ because there isn't a high pool to go off of and more then likely most Nami jungle games don't end well.
Dreadlocks1 votesThis has a lot of truth to it. If i get a bunch of kills on Shen, i find it really easy to get an S+ because they look at Shen as a tank and therefore shouldn't get kills very easily.
IcyPepper1 votesI don't think it's based off kills alone.
Although if you do everything good on Shen, and get above average kills, it should be easier. So I'm not saying you're wrong >.>
Dreadlocks1 votesForsure, but i wanted to stay on "master nami's" topic of doing something that your class isnt known for doing.
SeargentAnus2 votesHave extremely high kill participation around 65%+. Have good CS for the role you're in (support at a 30 minute game will probably need 60+ CS while Jungle will need something like 140+ and adc 200+). Have very low deaths less than 2 (2 is the highest I've gotten an S+). Just be aware it bases it off of other champions in that role, so you won't need to CS as well on a Janna support as you would need to on a Vel'kozby support.
Catastrop1 votesOh geez, I've already failed in a lot of these. I either went one death over, didn't have the kill participation or it's because of my general inexperience with these champions. Not to say, I'm a noob at them, but compared to the more die-hard mains, I'm probably lacking the skill they have with them.
Kaioko2 votesFrom my personal experience in mid I've had a much greater rate of S+ by playing a champion like Cassiopeia than let's say Annie. I think CS, Damage, KDAs, Wards all affect it but I'm starting to think champion difficulty (Cass has the highest possible at 100%) also plays an effect.
Best Valor NA2 votesDo you ward?
Catastrop1 votesA lot. Granted not as much as I'd like with Green Wards gone, but I'd say I do a lot of warding. I'd check it on Ward Score but there seems to an error on the site.
PUT U IN MY OVEN2 votesEverything
Catastrop1 votesWell Said.
Valderis Vandala2 votes"At the end of your game, you can hover over your Champion Mastery points bar to see the grade our new system gives you. These grades are awarded based on your overall performance in the role, the champion you're playing and, crucially, where you're playing them. For example, we look at an Annie going mid differently from an Annie going support. We take a series of performance metrics and compare how you did to all other players in your region in that specific champion/position combination.
This is percentile-driven; earning a high score means you performed in the top x% of players in that champion/position combination."
Quote taken from: https://support.riotgames.com/hc/en-us/articles/204211284-Champion-Mastery-Guide
Looks like it depends on your performance with a champion depending on what role you take as compared to other players playing that champion and role in your region. The actual calculation and metrics used are of course a secret so nobody can tell you what it is exactly you can do to get a high rank. But considering that they have to use different metrics to deal with the different roles (at least thats my guess*) it can't just be as simple as getting a good KDA or high CS, and because your performance is compared to other people in your region only the very top % of performances will get a S+ ranking.
*It is possible that the same metric is used to determine the performance of all champions and all roles, but I still doubt it would include things like KDA and CS.
Mat7itan1 votesYour CS and kill participation have to be damn near perfect. I've only gotten S+ once or twice.
MegaManYYZ1 votesif you play a less popular champion, you have a better chance at getting an S+ rating. The ratings are based off other performances by all the players with that champion.
Catastrop1 votesOh, well that makes sense now. I'd hardly call myself as good as some of the more experienced mains of these champions. Damn, the bars set so high....I'll just have to keep trying.
The Bearded Bard1 votesThe other day I went 17/0/6 with Shyvana jungle, had more minion kills than anyone on my team, a baron and three drags under my belt, 3k more gold than anyone on my team and 6k gold more than anyone on the enemy team all in a 24 minute game, but I still only got an S rank.
xJLGx1 votesbecause you think you do good when in reality you dont lol. I get s+'s all the time
Catastrop1 votesWell from what I've gathered from all the responses, I basically just need to get better and play on par with the really seasoned players who die-hard main the champions I play.
Drackolus1 votesI've gotten about 5 on 
. Really not sure how the grading system works, though it's definitely accurate. I suspect that you have to not only do well for your champ, but do well compared to your team. Just a suspicion though. The games I got S+ in weren't always my best scores in any aspect, but they were always ones I helped carry hard.
Catastrop1 votesWell, in the games in question, everybody did really well. No one really did better than the other.
JohnTheLibrarian1 votesWork to get assists on Drags, barons, ward more, get assists and kills as much as possible for participation, CS
Dasdi961 votesThis rating system is so weird. I once went 13-1-16 on trist and got only an a. Then I went 10-11-14 on trist with lower cs and I goy an s.
IcyPepper1 votesHow many objectives did your team get?
I'm sure it's based off more than just cs and kills. Maybe damage dealt is also a consideration? o.O
ValyrianBlade1 votesI've only gotten 2 or 3 S+s. I remember having the same feeling until I got my first one, then I got 1 or 2 in the next few games :o
I'm pretty sure you basically need: Over 70% kill participation Very few deaths if any Good CS score Map presence - you need to roam Lots of wards - change your trinket, always have a pink out, etc...
Basically just all around good play. Like an excellent game where you were going to carry regardless of whether your teammates fed early or not.
Catastrop1 votesHoly shit. I have my work cut out for me.
ValyrianBlade1 votesEh, it moreso comes down to a game where you just snowball early. More about getting a bad opponent than about being good yourself, unfortunately.
It's very easy to play well when you have a free lane and a solid lead in levels and gold.
seachrome1 votesI'd think the best way to get high ranks is to play unpopular champs. Since it compares your score with all other players in your region the less other players there are the more likely you are to get a high score.
Catastrop1 votesWell, I've been thinking about buying 
for a while now.
Asayake 5150glow1 votesI got one absolutely once.
I went 10/3/9. Roughly.
Or maybe it was the 20/2/14.
I can't see the scores on my match history ;(
Miku Lv991 votesFigure it's something like this, though I don't know the exact values:
Kill: +10 points Death: -4 points Assist: +3 points Every 10 cs: +1 point
Or something like that. And then you have to get about 2x the average for the champion you're playing in order to get an S+. Just keep in mind these are probably not all that accurate, but if you can get a bunch of kills with a support you can often get an S+, since supports have significantly lower average ratings than other champions. We might see an influx of supports playing other roles just because people see that as the easiest way to get a high rank so they can get these new rewards for getting good ranks they're going to add.
Jalkan1 votesI believe Riot said somewhere that grades are based on how well you did compared to other similarly ranked players who played the same champion in the same role as you. So if you play support Jax, you'll be judged on your performance against other players who played support Jax.
Worst Kench NA1 votesJust participate in lots of kills I guess? I'm sure objectives figure in there somewhere
IDK, the only times I've ever gotten S+ were as Soraka and Karthus.
RastaBrotha1 votesIve noticed a pattern on my S+'s on the stats page, basically whenever I got one I had:
Most damage against enemy champions (on carry type champions)
Having most of these usually garentees me at least an s- but I only ever play ad off-tank fighters :^)