The original 2099 universe started with four series: Spider-Man 2099, Doom 2099, Punisher 2099, and Ravage 2099. Next came 2099 Unlimited, X-Men 2099, and Ghost Rider 2099, followed by Hulk 2099. X-Nation 2099 and Fantastic Four 2099 both had brief runs. There were also a couple of one-shots dealing with Doom's presidency. When all the series were ultimately canceled, there was an eight issue 2099: World of Tomorrow series, and then a final concluding one-shot that wrapped it all up, 2099: Manifest Destiny.

Spider-Man 2099 is widely available, both digitally and printed. The series is available on Marvel Unlimited and Comixology. There are several nice collected editions available, out of which I recommend the Spider-Man 2099 Omnibus Volume 1, which contains most of the Spider-Man material covered here.

Doom 2099 is also easy to find, both digitally and in print. If you do not have access to Marvel Unlimited, the Doom 2099 Omnibus is a good choice as it contains the entire series, one-shots, and all eight issues of Fantastic Four 2099. Please note that about one-third of Doom 2099 is written by Warren Ellis, which is a dealbreaker for some people.

The other two of the original 2099 series, Punisher 2099 and Ravage 2099, are harder to find. One issue of each series is included in the Spider-Man 2099 Omnibus and the X-Men 2099 Omnibus as part of "The Fall of the Hammer" crossover. Those single issues are also carried on Marvel Unlimited. To read the rest of the series, you will need to track down the individual issues.

In terms of 2099 Unlimited, a few scattered stories show up in the Doom 2099 Omnibus and the Spider-Man 2099 Omnibus (Volume 2, this time). Three issues are available, at least in part, on Marvel Unlimited. For the rest of the stories, you will need to track down the original comics. Note that several stories in 2099 Unlimited are written by Warren Ellis, and all the Hulk 2099 stories are written by Gerard Jones — both men with concerning histories.

X-Men 2099 is easy to get a hold of, both digitally and in print. I recommend the X-Men 2099 Omnibus, which also includes X-Nation 2099.

There is a Ghost Rider 2099 Omnibus available, both in printed and digital versions. It contains the entire series and is worth your time.

There is no collected edition of the ten issues of Hulk 2099, and given the criminal history of its writer Gerard Jones, it is unlikely to ever be collected. Tracking down the original issues is your only choice if you really want to read it (and honestly, it is probably not worth your time).

As noted above, Fantastic Four 2099 and X-Nation 2099 are available in the Doom 2099 and X-Men 2099 Omnibuses, respectively,

The swan songs of the 2099 universe, 2099: World of Tomorrow and 2099: Manifest Destiny are not collected either, and you will need to track down the original issues.