Instead of Complaining About Individual Champs...
Instead of complaining about individual champs, let's talk about items. Right now, items are so incredibly narrow that the problem is rarely specific champions (cough 
anyone with "lee" in their name cough), and is usually more a case of "who can use item X the best?" Not every mage builds the exact same items, but mages are just about the only role is a good spot (fighters are in an acceptable place) right now in terms of item options. Even then, they could be improved.
From here on is gonna get fairly rant-y.
Let me first describe how I think offensive and defensive items go in terms of roles. This is a general statement, and the exact items vary from champ to champ, but the broad overview is that offensive items should be glass for damage dealers, and defensive items should still give AP, but much less, as well as giving defense of some kind (HP, armor, MR). For tanks, offensive items should give a slight amount of damage, while still giving some defense. I often consider defensive mage items to be offensive tanky AP items. Defensive items for tanks are pure tank items, and give no damage of any sort. This is also not including common core items, such as
,
, or
, which many mid laners need to get before looking at situational items.
Mages
Offensive Options: 

Defensive Options: 


Zhonya's is actually a poorly designed item, imo. The fact that it gives armor sets it as a defensive choice, but the fact that it gives 120 AP and takes a
Needlessly Large Rod is completely counterproductive to this point. Defensive items should never force a champ out of their role. Suggesting that a mid laner needs to get
or something similar just to lane someone is a joke. If they still need defense after getting defensive items, then it seems fair to me to say that they were outplayed. If a mid laner needs help against CC, there needs to either be something that gives them Tenacity at the cost of damage/effectiveness without completely setting them back just to survive lane (
Moonflair Spellblade (MFSB) is a perfect example of this. If only it was on SR), or
QSS needs to build into something with a Blasting Wand/Amp Tome.
All of that being said, mages're actually in the best spot in terms of situational builds.
Tanks
Offense: 


Defense: 



This actually isn't that bad, either. The problem is that none of these items synergize with each other at all (aside from being close to enemies with Abyssal/Frozen Heart/Sunfire, but that isn't a very strong identity for people with so much utility, "stand near bad guys"). Another problem is that Riot seems to think that tanks have this massive amount of health, and that giving tanks benefits from stacking health makes them extremely durable. Out of all of these items, only four give HP, and one them (Warmog's) gives you nothing BUT, making it a less than desirable choice on most tanks/fighters, aside from those with crazy health regen like Mundo, Garen, or Volibear. It also seems weird to me that tanks are forced to choose between armor/mana, and MR/HP. Sunfire's a fairly weak item from a tank standpoint, dealing decent damage at the cost of beefy stats. That leaves Omen as the only option that gives armor and HP. You're forced into it if you need both in one item.
Dedicated items for one/two things (Spirit Visage is about HP and MR, Frozen Heart is armor and mana, Warmog's is HP, etc.) should be stronger, but weaker, all in one options should exist. Example: Something that builds out of
+
, but gives less of every stat for those times when you really need everything to catch up, would help a lot.