Check PG's profile - he has 124701 (at the time of writing) and would be significantly off the top. He has deliberately removed himself from the listing.
This isn't really an answer to the question of influence, but it gives you a start on the question. There's the list of the top 100 people ranked by karma - that doesn't mean they're influential, of course.
Aight: How about "The capacity to have an effect on the character, development, or behavior of [the collective mind of the people who read HN] ...something like that.
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[ 3.2 ms ] story [ 43.6 ms ] threadhttp://news.ycombinator.com/leaders
which get submitted here (in my opinion some of the most insightful comments posted on HN are from his blog).
So, maybe you should define "influence".
* How would you measure that?
* How would you get everyone to agree?
Some people such as ColinWright post many influential articles.
Others such as Grellas [average comment score 38.8] are known for authoritative comments in their areas of expertise.
The most influential poster is PG.