These are people who made lots of contributions:

Tatsuhiro Tsujikawa <tatsuhiro.t at gmail dot com>
Ross Smith II <aria2spam at netebb dot com> (Windows port)
Nils Maier <maierman at web dot Germany>

The aria2 contributor's list extracted from commit logs [1]:

103yiran
Alexander Amanuel
Anthony Bryan
Artur Petrov
Athmane Madjoudj
Char
Cristian Rodríguez
Dan Fandrich
David Macek
Florian Gamböck
Fredrik Fornwall
ITriskTI
Igor Khomyakov
Jarda Snajdr
JimmyZ
Juan Francisco Cantero Hurtado
Kcchouette
Kurt Kartaltepe
Michał Górny
Michał Leśniewski
Mingye Wang
Nils Maier
ORiON-
ReadmeCritic
Ross Smith II
Ryan Steinmetz
Ryo ONODERA
Sarim Khan
Sergey Zolotarev
Sonny Piers
Sébastien Cabaniols
Tatsuhiro Tsujikawa
Torbjörn Lönnemark
Tse Kit Yam
Vasilij Schneidermann
Zoltan Toth-Czifra
amtlib-dot-dll
c3mb0
diadistis
geetam
gilberto dos santos alves
gt
klemens
kwkam
luokar
mozillazg
multisnow
oliviercommelarbre
qweaszxcdf
rotor
suzker

[1] https://gist.github.com/tatsuhiro-t/deaffeb064652104ad11