Okay, just going to go over some things. You are 8-17, so 25 Ranked Games... not a lot for me to go on. But, I'm here to try to help.
We'll just take the Champions from Top to Bot on your LOLKing, which brings us to:
Nasus. 4-5, 44.4% Win Rate. 1.4/6.7/5.0 (0.96 KDA). 132.8 CS per Game.
First thing, this is your 'best' Champion, as far as can be told with the limited number of Games you've played. So you are ABSOLUTELY CORRECT to have this ALSO be your Most Played Champion in Ranked. You are trying to stick with your 'Strong Pick', and that's always the best bet.
Next up, your KDA. 0.96 is what I would deem Bad on any Champion, it's Sub 1. (For reference, I call 2.00 Good, then generally scale up for each 0.25 added. 3 and 4 KDA are very solid over time.) HOWEVER, some Champions rely much less on KDA to be effective, and Full Tank or Sheen to Tank Nasus just so happens to be one of them. So... don't worry about the KDA here. You are very close to 1, which means each time you Die you return the Kill. This is PERFECTLY FINE, as long as you are getting other things out of your Lives (Turrets, Buffs, Neutral Objectives).
The major problem I see with your Nasus is you are Under-Fed. You don't have enough Farm, so you are Starving. Without seeing the actual games, you may actually be playing well within the Team but simply not have enough Items to finish the plays... thus the way higher Assists than Kills (Nasus is an Executioner with his Q). Also, the Lower Farm naturally results in Less Q Stacks... and THIS is an issue in any Nasus Build (to include AP Nasus... stacks STILL matter, but that's Off-Meta so let's stay with the basics).
So this tells me you are grouping way to early and leaving your Farm before you're actually strong enough to matter... taking guys down to low health but unable to finish... while dying in the process and being left to hope your Team can clean it up.
As Nasus... and in fact many Top Laners... you simply MUST Farm. It's why so many take Teleport... to TP back after First Buy so as not to lose ANY CS, to TP to a Team Fight as needed (once strong enough to matter... if not, stay and push) or to TP back to Lane after Grouping from Fountain for an Objective like Dragon or a Turret Push.
Nasus is a Split Push God, your Team should EXPECT you to very rarely be with them. Which means it is YOUR job to have more Vision than usual, to ensure you aren't being closed in on to be picked off. I'm willing to bet more than a few of your Deaths come when you FEEL like you are doing a great thing, wailing on a Turret, and suddenly 4 people insta-gib you.
So long/short of it... Farm More, get More Q Stacks, only Group once YOU are Ready, Vision, Vision, Vision, Vision, Vision and when you think everything is Warded... add one more Ward. Do NOT let your Team pressure you into Grouping until you are going to matter, because an Under-Fed Nasus HURTS his Team when grouped. Much better to stay Top, Farm and Push that Turret. After all, even once Fed... you'll still primarily be Split Pushing.
Keep at him, work on those things and I'm sure you'll get both the KDA and the Win Rate% up in no time... and even more Objectives, which is what REALLY matters as a Nasus.
Volibear 2-6, 25% Win Rate. 1.6/10.1/4.8 (0.65 KDA). 71 CS per Game.
So right off the bat, one single number jumps off the paper. 10.1. That's a lot of Deaths to Average. However, I'm here to help... not bash. So let me try to explain WHY, based off the numbers, you die so much with Volibear.
First off, Volibear only has ONE Play Style. Balls to the Wall. That's it. There's only 2 Gears on his Shifter... Parked (Dead) and Hyper-Drive (Alive). That's it. Voli has no 'Passive Game'. So let me first start with a compliment... your INTENT on how you play him is correct. You likely die when you wade in to dead and can't get out... the wading in part is correct, I feel it's a combination of not knowing his (and your) Limits, and being (again) Under-Fed.
Much like Nasus, Volibear is a Champion that can get huge bang for the Defensive Bucks... you can absolutely build him Full Tank (though better builds are out there, Full Tank is the safest to learn on) since he's still going to pack a punch. Also, Volibear can ABSOLUTELY Carry... but when starting out you should see yourself as a Front Line Tank, there to soak up damage and toss Squishies. As you get better with him, expand the build and the carry potential... but first learn to be that Unkillable Thing that can hurt.
So... how do you become that Unkillable Thing? Gold. But the good news is, it doesn't take a lot. Voli has a Passive that gives him huge Regen when low... so, Spirit Visage. This has been built in every Voli Build I've ever seen... if you aren't building it, start so immediately. Again, with the Regen, we have Warmog's. Less valuable in Team Fights (aside from the huge HP) as it ONLY Regens when Out of Combat... but as Voli often you will dive in, do your damage, and walk out. So if you need to Walk Out and rely on Regen anyways... then waiting for your Mog's to proc shouldn't be an issue and you run back in with 100% Health after like 12 seconds (thanks to Visage helping it). Boots depends... in a Heavy Marksman comp (Enemy team) Ninja Tabi could be useful... BUT the most danger to a Volibear is CC, so Mercs or Swiftness. Speaking of CC, Banshee's is great. Which leads to an item most don't get to use since the Nerf... Thornmail. Eventually they have to Auto Attack you (Randuin's, Dead Man's Plate can fill in for Thornmail on Voli as well).
Once you learn more about him and improve the skills, lots of other Build Options open up to magnify his Damage... but to deal Damage we must first learn to Stay Alive.
That's all I have time for right now, will check in later.
I myself have many Champs I'm bad at, and I started Ranked with 16 Champs and no Tier 3 Runes so I know the pain of getting in to deep to soon. I'm not calling out or bashing in any ways, and it's encouraging that it SEEMS like you are at least playing the Champions in the right spirit.