I am looking for what is the rule on Champion Fighter "Improved Critical & Superior Critical". Is the roll of 18 & 19 considered an automatic hit like the roll of 20. 

You know, I hadn't considered that this might be fuzzy at all, but I can see why it would be to some. Let's go to the text from the PHB under Combat.

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. This is called a critical hit, which is explained later in this chapter.

 

So this tells us that a natural 20 is synonymous with a critical hit. Then we go to Champion...

Beginning when you choose this archetype at 3rd level, your weapon attacks score a critical hit on a roll of 19 or 20.

Starting at 15th level, your weapon attacks score a critical hit on a roll of 18–20.

So because of the wording, they would be auto-hits, because all the rules of a critical hit apply.

As a note, I didn't include the later text about critical hits because it only talks about damage and isn't relevant to the attack roll.

Agreed, critical hits always hit.

Yes there are exceptions, but generally speaking they always hit so Champion 19+ or 18+ always hit.

Arguably yes, and arguably no. Athanar explained the reasoning for why it would. The reasoning for why it possibly wouldn't is semantics. The rules explicitly call out rolling a 20 as an automatic hit, and that doing so is also a critical hit. It then goes on to explain how damage on a critical hit works. It does not say that a critical hit is an automatic hit, just a 20. They can possibly be interpreted as being separate, distinct effects. Under such an interpretation, rolling an 18/19 (with the relevant feature) is a critical hit, but must still beat the target's AC.

• Rolling a 20 is an automatic hit
  • Rolling anything else is not an automatic hit
• Rolling a 20 is a critical hit
  • Rolling an 18-20 is a critical hit with certain features

We often cite the mantra of "rules do what they say they do, and no more or less". The rules say that rolling a 20 is an automatic hit. They do not say a critical hit is an automatic hit. The order of premises in a logical proof matters. The Champion feature also does not say that an 18-20 is an automatic hit.

Personally, I lean towards the opinion that any critical hit should also be an automatic hit. It's just easier to deal with, and it's certainly more fun for the players. As far as practicality goes, if you roll an 18 or 19, it's unlikely that you're not going to hit after adding modifiers anyway. Like most things in 5e, they could've written it more clearly than they did.

I would rule that it's an auto hit. I mean, can you just see the deflated look on the player's face when he rolls an 18 and shouts, "I crit!" Then you tell the poor soul, "Nah, it's a crit miss."

A critical hit, as the name mention. always hit. The rules say that "If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit" and the feature let "your weapon attacks score a critical hit on a roll of 19 or 20."

While not official ruling, the Dev also answered in this direction on numerous times;

@_Blackstorm Does a fighter with improved/superior critical auto-hit on natural 19/18+ or still only with nat 20?

@JeremyECrawford If a feature says you scored a critical hit, you hit.

@20771646 Are Fighter Champion improved critical hits like a normal 20?

@JeremyECrawford Yes, they are.

Quote from Sigred >>

Arguably yes, and arguably no. Athanar explained the reasoning for why it would. The reasoning for why it possibly wouldn't is semantics. The rules explicitly call out rolling a 20 as an automatic hit, and that doing so is also a critical hit. It then goes on to explain how damage on a critical hit works. It does not say that a critical hit is an automatic hit, just a 20. They can possibly be interpreted as being separate, distinct effects. Under such an interpretation, rolling an 18/19 (with the relevant feature) is a critical hit, but must still beat the target's AC.

• Rolling a 20 is an automatic hit
  • Rolling anything else is not an automatic hit
• Rolling a 20 is a critical hit
  • Rolling an 18-20 is a critical hit with certain features

We often cite the mantra of "rules do what they say they do, and no more or less". The rules say that rolling a 20 is an automatic hit. They do not say a critical hit is an automatic hit. The order of premises in a logical proof matters. The Champion feature also does not say that an 18-20 is an automatic hit.

Personally, I lean towards the opinion that any critical hit should also be an automatic hit. It's just easier to deal with, and it's certainly more fun for the players. As far as practicality goes, if you roll an 18 or 19, it's unlikely that you're not going to hit after adding modifiers anyway. Like most things in 5e, they could've written it more clearly than they did.

 Funny thing, I would have brought up the possibility presented here, except the rules don't say it's ALSO a critical hit, they just equate the two events. It's a small difference, but small differences can matter.

 TBH If the party is fighting a creature against which they miss with 19's, they will need all the help they can :)