Paladins are completely OP in almost every way imaginable.
OK let's drill down on that.
I don't think anyone cares about this. They're a fighter type. Pretty much all the classes can achieve a good armor class if they want, and get the at-will damage they want. No biggie here.
auras that grant Charisma bonus of up to +5 on all saves. Other major aura bonuses such as 1/2 damage on spells.
I agree the save bonus at level 6 is meaningful.
Smites that go off twice per round for mega damage that far exceeds what spells of appropriate level can achieve and they can call those smites after they roll and on crits for double damage. Smites are so OP that many paladins in my games don't bother to cast divine spells as well and just smite like crazy.
A few claims in here. First, smites that go off twice per round. It's a limited resource applicable only to when you hit with a weapon, which isn't by definition twice a round. It is limited by spell slots as well, and you don't get a whole lot of spell slots. The actual damage from the smite isn't itself more than what spells can do, and in fact it's on average less from the smite itself. For the obvious example, a 3rd level spell slot used for a smite is 4d8 (average 18) and to only one target. A fireball of the same spell slot is 8d6 (28) and to many many targets. Even if the targets save, they're still taking almost the same damage as the smite (which again is only one target).
As for crits, they're quite rare, and usually already over-damaging the target beyond their hit point total.
Now you still have the weapon damage. But it's pretty ordinary weapon damage, which you had to hit with. And as you don't have a lot of the special abilities of a ranger or extra feats of a fighter, you're likely not hitting as often as they do.
Add in the additional D8 for damage rolls they gain
What now? You mean from higher level spell slots? As mentioned above, that's not actually a very good payoff for those higher level slots sometimes.
and the conditional D8 for fiends & undead and they are completely overpowering all martial characters even the rogue in the damage department.
I disagree. The rogue can do it all day. The Paladin can only do it a very limited number of times. They really don't get many spell slots. You're basically saying a Paladin can nova better than a rogue, which is true. But, so? A rogue can outlast them. So, seems pretty even to me.
Especially if you allow overpowered feats like great weapon master to factor into the equation. 5E really dropped the ball on class balance issues with the paladin. They are far above almost every single class in what they can achieve and really step on many other classes toes (fighter, rogue, cleric, wizard just to name a few)
I really have no idea why a Paladin steps on a cleric or a wizard. That's a very apples to oranges comparison that would need a lot more support.