I've tried to break up the list a bit, mostly whenever one of the three main events cuts in and I've included Non-Spider-Man books.
Peter Parker - The Ultimate Spider-Man
The Marvel Team-Up issues aren't by any means essential, but they do detail his first encounters with a lot of characters that later on turn up in the main book. As such I would recommend reading them, just so you have some idea of their relationships with him. I would also recommend reading the annuals as well. Certain things in the main book don't make sense if you skip them. Suddenly dating Kitty Pryde being a big one.
USM (Ultimate Spider-Man) 1-13
UMTU (Ultimate Marvel Team-Up) 1-8
USM 14-21
UMTU 9-16
USM 22-27
USM Super Special 1
USM 28-46
Ultimate Six 1-7 (This is USM in everything but name. Essential reading)
USM 47-85
USM Annual 1
USM 86-105
USM Annual 2
USM 106-120
USM Annual 3
USM 121-129
Ultimatum
Ultimatum is generally considered to be a bit shite. And with good reason. However it does have a few things in it that change the universe, so I would grit my teeth and hammer through it.
Ultimatum 1
USM 130
Ultimatum 2
USM 131
Ultimatum 3
USM 132
Ultimatum 4
USM 133
Ultimatum 5
Ultimatum: Spider-Man Requiem 1-2
USM (Volume 2 - Issue numbers reset to 1) 1-15
USM (Volume 2 continues, but reverts back to original numbering for 150th issue) 150-160
Ultimate Fallout 1-6 (These, plus USM 150-160 are probably my favourite run of issues in USM. Utterly essential)
UCSM (Ultimate Comics Spider-Man) 1-12
Spider-Men 1-5
Divided We Fall/United We Stand
I've included all the X-Men and Ultimates issues for this event as well, because UCSM doesn't really explain what's going on very well. The upside being that it's not a huge event and I enjoyed it.
UCX (Ultimate Comics X-Men) 14
UCU (Ultimate Comics Ultimates) 13
UCU 14
UCSM 13
UCSM 14
UCX 15
UCU 15
UCSM 15
UCSM 16
UCU 16
UCX 16
UCX 17
UCSM 17
UCU 17
UCU 18
UCX 18
UCX 18.1
UCSM 18
UCU 18.1
UCSM 16.1 (Fits better after DWF/UWS)
UCSM 19-28
Cataclysm
Again, it's not a massive event, but it changes the world. Hence I've included the X-Men and Ultimates Issues as well.
Cataclysm 0.1
Cataclysm: Ultimate Spider-Man 1
Cataclysm: Ultimate X-Men 1-3
Cataclysm: The Ultimate's Last Stand 1
Cataclysm: The Ultimates 1
Cataclysm: The Ultimate's Last Stand 2
Cataclysm: The Ultimates 2-3
Cataclysm: Ultimate Spider-Man 2-3
Cataclysm: The Ultimate's Last Stand 3-5
Ultimate Comics: Spider-Man 200
Miles Morales: Ultimate Spider-Man 1-12