I usually go by the rule that the first to insult is the childish one.
Personal opinion is a little inherently in most arguments, but it's not like my opinion isn't supported.
You're right that the question is Which class is the Best Caster, however we need to establish parameters to what makes the Best Caster, the Best.
Now to be the Best Caster one would assume you have...
-Strong Spell List
This is easily one of the most important factors. Since this is what determines the effect of our Caster will have. Having a large selection of spells is crucial, but so is having the arguably best spells readily available. We need both Diversity and Power.
-Strong Caster Features
It's often not enough to just having access to good spells, since a lot of classes may also have access to those spells. It's also crucial to be able to manipulate those Spells or to provide additional support thru your Caster Features.
If you have additional criteria that you feel is crucial to being a strong Caster, feel free to add it.
Now each of these core necessities have different sub importance.
Spell List might be weighed by Damage, Control, Support, etc.
Needless to say its obviously a pretty complex system.
However in the end...
Wizard has the best Spell List and among the Best Spell Casting features.
Cleric and Bard have great Spell List. Cleric has better Spell Casting Features, while Bard has unique mechanics that allows them make their Spell List uniquely the best in a limited fashion.
Sorcerer Meta Magic is probably the 2nd or 3rd Best Spell Casting features with Metamagic, but the more you examine Metamagics the more you realize that a lot of them are fairly limited in scope. You're often limited to only a few amazing options, and a huge amount of mediocre and poor ones. Sorcerer is then further limited by a pretty Terrible spell list.
Druid is limited by having nearly zero Spell Casting features, and a pretty terrible Spell List that inherently misses on a huge amount of crucial and powerful Spells. They get a few useful unique spells, but lose too much to make them worth it.
So
Best Spell List
Wizard ~ Bard
Cleric
Sorcerer
Druid
Best Spell Casting Features
Sorcerer
Wizard
Cleric
Bard
Druid
Oh, and I'm not hiding behind "Pre-Xanthar". I'm acknowledging Xanathar changes a lot of the previous power levels. Which means we need more time to fully understand what Xanathar brings. Trying to claim rankings before it has been properly analyzed is just Childish.
Finally, if you didn't feel it was worth it to respond, then you shouldn't have responded... Twice
Edit - Disclaimer which should go unsaid but I'm starting to doubt it... Being the Best and the Worst is something you need to consider what it means. Being the Best Caster and being the Worst Caster doesn't mean that the Best is overpowered, and the Worst is unplayable. All 5 options are incredibly powerful and can be game changing in any game they are in. All 5 options actively switch places on specific situations that might make one better than the other despite their general ranking. However, in the end you need to stop and consider who is the most useful for the majority of situations. Kinda of like average as many different situations you can think of. In the end the one of the most variety of tools and assets will generally rise to become the Best Overall. That is what we have here.
Agreed