"There is no doubt this is arguably wrong"

Solution 1:

Fairly obviously there can be multiple X's, any or all of which may be 'arguably' the best X. To qualify for that designation they only have to be capable of being argued for – they don't need to actually be the best.

Equally obviously there may be some X's which are so bad they're not remotely capable of being considered for 'best', and some other X's that are so good no-one would deny they're at least in contention, even though only one can actually get the top rating.

So – strange as it may seem on first reading – there is no doubt OP's sentence is both grammatically and semantically sound. Though we still don't know whether the product in question really is the best - that needs to be established by argument.

Thanks to @Peter Shor for pointing out what seems an eminently plausible rationale for the unusual phrasing. The writer intended inarguably. There's still the tautology of inarguably repeating the sense of There is no doubt, but pointless repetition is a feature of much "persuasive" writing – particularly in advertising (and product reviews, which are often incestuously related).

Solution 2:

This is grammatically correct, though I imagine the strict meaning is not the original intent. To say "there is no doubt that x is arguably y" means that there is no doubt that someone can make an argument that x is y.

For example, "There is no doubt that Casa Juan has the best burritos in town" means that nobody would doubt that in a contest for best burrito, Casa Juan should be one of the finalists. You could not say "There is no doubt that Taco Bell has the best burritos in town" because nobody would seriously consider Taco Bell to be even a contender for the best. (FWIW, I really like Taco Bell :-)

So the lack of doubt pertains to whether a serious argument can be made, not whether it actually is the best.

Nonetheless, if this is an advertising thing it probably the original intent we "we are going to tell you it is the best, but put a little equivocation in there in case we get sued." If it is a user review, it is probably just a fat finger.