Big list of web game portals / sites that accept games mainly from Flash, HTML5 and Unity 3D developers:
Facebook Alexa rank 3
Y8 Alexa rank 761
Kongregate Alexa rank 1375
Miniclip Alexa rank 1750
Kizi Alexa rank 1816
Crazy Games Alexa rank 2292
Newgrounds Alexa rank 2624
Poki Alexa rank 2985
itch.io Alexa rank 3449
Armor Games Alexa rank 3472
A10 Alexa rank 4164
Pogo Alexa rank 4955
Big Fish Games Alexa rank 5113
AGame Alexa rank 6009
Game Jolt Alexa rank 6537
Addicting Games Alexa rank 6657
Paco Games Alexa rank 8024
Girls Go Games Alexa rank 9280
Zepak Alexa rank 9516
Ninja Kiwi Alexa rank 10681
io Games Space Alexa rank 11295
NotDoppler Alexa rank 14516
Games Games Alexa rank 15194
Yepi Games Alexa rank 21526
Nitrome Alexa rank 24779
KHBG Games Alexa rank 24690
Game House Alexa rank 28541
Free Online Games Alexa rank 28628
Games Freak Alexa rank 35802
MAD Alexa rank 36964
Box10 Alexa rank 43227
One More Level Alexa rank 48128
Mouse Breaker Alexa rank 52226
Games 2 Girls Alexa rank 54837
io-games Alexa rank 56104
Fun Brain Alexa rank 57127
Frip Alexa rank 60455
Puff Games Alexa rank 65496
Crazy Monkey Games Alexa rank 68705
Flash Games 247 Alexa rank 72799
Game Gape Alexa rank 76718
Mind Jolt Alexa rank 78550
Bubble Box Alexa rank 80294
Cool Games Alexa rank 90502
Zapak Alexa rank 95136
GameZ Hero Alexa rank 109726
Spil Games Alexa rank 152091
Game Garage Alexa rank 1295785
* Alexa rank accurate as of 2nd October 2017