Tostiproeftest still underway. The local critical favorite is De Caulils Tosti, and if there are no limits on how you trick your sandwich out, then yes, strong contender. It's like a fondue sandwich. I would've added a pinch of salt, but that's me.
In the "non-gourmet" class, my Amsterdam favorite is Gambrinus (below). Note the Heinz ketchup, the only appropriate ketchup to serve with a tosti. And notice that there's enough cheese so that it can't quite contain itself once bitten into. This is what you're looking for.
This is an often-NSFW, mostly gluten-free kitchen notebook that also occasionally threatens to turn into something else and fails, thus remaining its same old cryptic and superficial self. These posts begin to fail to explain (start at the bottom).