The best ever question with Van Dijk is a question of whether you look at achievements or whether you look at attributes/effectiveness. Take Baresi, for example. He played much of his career in Cappelo's more defensively-minded Milan side, in a team that was probably the biggest spender in, at the time, the biggest spending league. Van Dijk hasn't had the opportunity to show his longevity at the top, because for some reason the entire scouting industry dropped a collective bollock in not realising that a 6'4" speedster with skill to burn and a glacial temperament was not, in fact, taking a punt on for the pennies he would have cost to buy from Gronigen. That's not something in Van Dijk's control.
But if we're talking attributes, who stands against him, really? Ferdinand was mentioned above as the best in the Prem - and it's a fair shout. Ferdinand was a great defender. But what would you say his best attributes were? His pace? His reading of the game? His athleticism? Van Dijk is faster and leads the best offside trap in the modern era - one that replicates the kind of aggressive, high-line that Baresi used to lead but in an era where most teams have given up on it due to how difficult it is to implement and the extreme consequences for even fractional mistakes. Van Dijk is better in the air than Ferdinand was, scores more goals, and is better on the ball. So how can Ferdinand possibly be considered a better defender when there's nothing he did well that Van Dijk doesn't do better?
Take Cannovaro, also mentioned above. Absolutely phenomenal player, no doubt. But his biggest strengths of leadership, positioning and reading of the game were, at best, only fractionally better than Van Dijk's. Stacked against that Van Dijk is faster, significantly stronger, better on the ball and in the air it isn't even remotely close.
And this is the thing with Van Dijk - you can name any of the greats you like, and their best attributes are comparable to Van Dijk's. Certainly some excel what he can do - there have been faster defenders, defenders with better passing etc. But they're not in a different ballpark - Van Dijk can compete with the best attributes of *any* of the all-time best, and then brings to the table 3 or 4 key attributes where he makes absolute mincemeat out of them.
There are few better aerial defenders than Hyppia, say - Van Dijk matches him in the air, and destroys him on the ground. Ramos has a shout for his ball skills, leadership and pace, but they're not better than Van Dijk's, and again Van Dijk beats Ramos in the air and has a much, much better temparament (has Van Dijk ever been sent off for us? No. How about yellows? He doesn't even miss games through collecting 5 in a season).
You can go on and on like this.
Another illustration might be that you could play Van Dijk in any system, and he'd be great. You could put him with any partner, and he'd be able to complement that partner's weaknesses - just look at how he plays with both Matip and Gomez (a pair who, if VvD weren't here, would be the kind of partnership you'd normally dream about).
Van Dijk is to defending what Messi is to attacking. He's beyond a generational talent and he'll be the benchmark to which future defenders are compared. And we managed to get not just him but Alisson in exchange for one Coutinho - absolutely the steal of the century, and also, incidentally, what moneyball and FSG's approach has always been about. Not spending less - but exploiting weaknesses in the market, in this case that defenders and goalkeepers are grossly undervalued compared to attackers.
