So I think we need to define terms. To put my professional hat on for a minute :)
I think 'the spark' and 'chemistry' have been used interchangably but I'd argue they're too different things.
From my experience, dates come in 5 levels of goodness.
Level 1 - actively unpleasant. Rude, arrogant, gropey, smelly. Wish I'd stayed home and watched paint dry instead
Level 2 - not my type. Didn't fancy, no fault of their own. A bit 'meh'. Wish I'd stayed home and watched that documentary about penguins*
Level 3 - potential. They're not bad looking, the conversation didn't have long awkward pauses, they seem quite nice. I think I may see this one again if they ask. Glad I got out of the house, I'm beginning to find penguins attractive.
Level 4 - Chemistry. Long looks, really quite fancy them, want to see them again. Possibly end up with lots of snogging. Sod penguins.
Level 5 - the Spark. Laughing with each other, there's chemistry there, where have they been all my life? Damn I wish we'd met years ago, we just 'get' each other - even my stupid jokes. I want to marry them and adopt a penguin together.
I've only had a couple of level 1s, most have been level 2, with a few level 3s thrown in. The Artist was level 4. Buffy was 4 and a half.
There, the Penguin classification of Dating according to Bant.
*substitute Peacock for Penguin if you are so inclined