For anyone you choose on the basis of making a special contribution which nobody else could have made, you might just as well choose their mother[*]. This is tedious, so let's close the door on that. What's needed is a scenario where the assassination itself stunts technology, and not the mere absence of the person assassinated.
What event could realistically prevent Europe[**] from crawling out of the
dark ages high middle ages and blessing the planet with the Reformation, Enlightenment, capitalism, the Industrial revolution, globe-spanning empires, the industrialization of slaughter through world wars, and all the other good stuff technology-enhancing stuff we're trying to get rid of?
If you are of a dramatic turn of mind then kill Martin Luther, as close as you can to the precise moment of his excommunication by Pope Leo X in 1520 and in a public place. If the ray gun doesn't produce enough sound and light, put on a bit of a show to make sure everyone gets the point.
The Catholic Church now has an objective demonstration that the Pope is is right and everyone else can shut the hell up[***].
Of course there's no putting the genie of European Empire back in the bottle by killing one person. And it's not like Catholics never invented anything -- of course they did, a lot. But empire operated under the assumptions of pre-reformation Papal authority is a very different and less vibrant thing than empire (even Catholic empire) operated under the religious/intellectual/political contest between Catholic and Protestant. You don't need progress when you have certainty.
Furthermore, a contest between Catholic Europe and (say) Chinese or Japanese empire just results in Europe winning due to the (albeit not massive) edge it already has. What's needed for the most technologically-productive conflict is schisms within the most technologically advanced culture, at a time of empire-building. So you have to stop Protestantism, and although it's still extremely difficult, your best chance is not just by deleting one person, it's by making a statement.
Couldn't there be conflict within Catholic Europe? Of course there can be and there was. But look at how Spain and Portugal divvied up the Americas almost politely and mindful of their obedience to monarchs and the Church. We attribute rapid technological progress to big conflicts of ideas.
[*] well, I suppose unless you choose someone born before 16AD and assassinate them after 16AD. But you get my point.
[**] I will point out that opinions vary here. In The Years of Rice and Salt Kim Stanley Robinson wipes out the whole of Europe, 99% of white people, in the Black Death, and still doesn't much retard modern technology. We can of course question his historical insight, or say that his alternate history is intentionally allegorical and therefore artificially similar to ours, but the Locus award and nominations for the Hugo, BSF and Arthur C. Clarke awards all say that it'll do for fiction!
[***] plausibly enough for fiction -- of course I'm being somewhat flippant, since there are political inevitabilities that potentially could drive a schism from Rome regardless of the odd punitive miracle. It's not as is Luther was the only person ever excommunicated for defying Rome, so with only one shot you're riding your luck whether it'll serve as a deterrent or not.