Sipa is the only person still working on Bitcoin on that list, and is therefore the longest serving programmer in Bitcoin development. So Sipa has probably contributed to Bitcoin development more than anyone else.
I'm pretty sure that sipa was only added to that list so that he could help migrate/maintain the bitcoin-dev mailing list. Development had moved from sourceforge to github in 2010 before the sourceforge repo was abandoned.
IIRC all of Wladimir, Pieter, Greg, Gavin, Jeff, and Matt all got involved within months of each other in 2010-2011. I think Wladimir is actually the longest contributing active contributor.
The person who contributed the longest and is still somewhat active is actually Dooglus.
A good place to find this kind of history is to scroll through the git log in reverse (i.e. earliest commit first).