League of Legends (LoL) Question: What method is best for getting chests?

There are a couple of different things you can do to increase your chances. The idea is abusing the system in some way to increase your odds of getting an S, but the system is very robust and it's not well known exactly how it works, just some generalities.

-Play with friends. If there is a difference in skill it's even better (because it's more common for the better player in the team to do exceptionally well), but just playing with more people improves your chances, since any S in your premade gives you a chest. Even random players you add just to play one game work for this.
-Play the champion in an off-meta role and do well. The rankings depend on comparing yourself to others and it considers the position you play, so playing a champion with less people playing (and most of them playing really bad) makes it easier to get S with some champions.
-Avoid getting killed. The difference between a 10/5/X and a 10/2/X KDA is three misplays, and ending with a very low death count pretty much ensures you get an S. It's pretty common to think a kill for a kill is worth it, but it's not if you're trying to get an S.
-Play well in general. The score takes in account farm, CC and vision score too, so make sure to use your trinket, buy control wards and keep farming until the late game.
-Don't relax if there is someone else doing well in your team. The score also takes in account if someone else is doing really well too. Relaxing and being carried often leads to an A+ even with good scores.

In the end, your best option is playing in a premade group. I understand it's not a solution for everyone, but it's by far the easiest way to increase your chances of getting a chest. Other than that, I found that focusing on not dying as much as possible can give you an S- even if you end up losing and having little participation (unless you get carried too hard).

My favorite technique is to use ARAM. Get a ranged champion, it'll be a lot easier. For runes, you'll want to take Domination. Ravenous Hunter and Taste of Blood make you hard to kill, while your keystone provides you with heavy damage. I prefer to take Dark Harvest, but Electrocute works as well. For Secondary runes, it depends on the champion, but I find myself usually taking Manaflow Band and Gathering Storm for the sustain and scaling.

In game, you want to play as safe as possible. After about 3 or so deaths, you're unlikely to get a S, so use your sustain and play as safe as possible. If you picked a ranged champion, you're going to want to focus on poking and using your sustain to survive all counterattacks. As a melee champion, it's going to be tougher. You'll want to target low health champions, and then escape to regain the lost health. If you took Dark Harvest and/or Gathering Storm, you'll be scaling without having to die. That will make it a lot easier to keep up with the people you're killing.

If you're a mage, you almost always want Luden's Echo & Rabadon's Deathcap for the damage output. As a ADC, you'll want Infinity Edge and a build that allows you to deal the most damage with one autoattack, as prolonged fights tend to get you killed. As a tank/bruiser, you're going to be poked down. Build Death's Dance and sustain to keep you alive.

No matter who you are, if the enemy team is all/almost all one damage type, build the appropiate defensive stat. Zhonya's and Banshee's Veil still provide AP. Wit's End, Maw of Malmortius, and Mercurial Scimitar provide AD/AS and Magic Resist. AD and Armor is a bit harder, but you can usually find something right for your champ. (Bonus points if you edited your runes to get the appropriate defense beforehand.)

That's generally how I do it. That are some small tips I would give. Make sure to ping your teammates when you want to use a Health Relic, and then wait for them. Doing this makes it much more likely they'll do the same for you, and have your back in fights. Take defensive summoner spells to keep you alive longer. (I recommend Heal, but take Barrier if your teammates already have a lot.) However, as a melee champion, take Mark. This can proc Dark Harvest, Taste of Blood, etc. It also lets you get to the target you want to fight, rather than the tank waiting for you to engage. Lastly, make a list of the champions you have chests for. It allows you to know which champion to take, and protects you from the feel-bad moment of getting a S on a champion you already have a chest for.

Well, last season you could very easily farm chests in Twisted Treeline map, thanks to low playrate - only new players focused on that game mode, making it brilliant tool for more experienced ones to harvest those sweet S+'s.

As for now, I can't really tell If there is any superb technique rn :/ What I would recommend you though is to expand your champion pool. More champions you enjoy and know how to play - more chests you are able to get. If you are finding some champions uninteresting, consider playing them 'differently' using different and perhaps more fun strategy.

Last tip (kind of a jerk one XD) I can give you is to be active on Mobafire. Who would ever need chests If you could select skins you want? All it takes is few comments, some basic guide, a tierlist and of course some luck. Or at least try out the Weekly challenges - you basically win an epic skin worth of RP just by playing League!

just play a sup and dont die lol

Umm. One way to get one chest at least was to go blitz top. You basically always get an S-S+. If you got friends get them to do it while u play some champ u haven't had a chest on yet.