He is without question the best central defender I've ever seen play for Liverpool
Same here.
In terms of best ever defender I'm not sure you can say that yet. Based on his all round attributes, and general lack of weaknesses, you can make an argument that he excels in certain areas compared with some of the great defenders of the last 30-40 years (Baresi, Maldini, Hierro, Nesta, Kohler, Cannavaro, Desailly, Ferdinand, Stam, Thuram, Puyol). I think there's a strong argument that's he better than some of those already.
What a lot of those great defenders have over Virgil is longevity at the top level. Van Dijk has really only been playing at top level club football for 2.5 seasons. He's only been an international for 5 season, has 30 odd caps and has never played in an international finals tournament (unless you count Nations League). He's got time to change this at both club and international level. If he keeps up this level for the next 4-5 seasons then he truly will be regarded as one of the very best few defenders in history. At the moment that shout is too premature for me.
In terms of Premier League defenders (so since 1992) I think he has a shout of being the best ever. Maybe already is. Excluding van Dijk, I think Terry, Campbell and Ferdinand are the best 3 centre backs in this era domestically. I actually think Ferdinand is the best out of those 3 due to his all round ability. VvD easily sits in a bracket with them 3 and I think you can easily argue he's 2nd in that list, maybe even 1st. He's already surpassed the likes of Adams, King, Desailly, Carrgaher, Hyypia, Vidic, Carvalho in terms of their Premier League performances.
Best ever Liverpool defender I've seen? Easily.
Best ever defender? Not quite yet