W/O knowing your reasons for wanting to get back in, it's hard to say what will best suit you.
Some points to consider:
* Do you want to use GW rules?
While the majority of 40k players use the latest official rules and codexes, many individuals play in the GW 40K-verse with alternative rules, such as Tomorrow's War, Stargrunt 2, or any number of other rules. Likewise there is a fairly large and thriving community of players who are using older rulesets.
If you are joining an established group, that will likely not be something you can choose, but if you are able to choose, you should consider all your options
*What style of play are you interested in?
A few powerful models (e.g. Grey Knights)? A large number of models (e.g. Tyranids or Orcs)? A force that emphasizes mobility (e.g. White Scars chapter of Space Marines)? Do you want tanks & artillery (e.g. Imperial Guard)?
*Does visual appeal and/or fluff matter to you and if so how?
Some enjoy playing a force of Tau for their animae-esque appearance, others like them because they are the closest thing to a "good" faction in the 40k-i-verse. Some enjoy playing Chaos Marines or Dark Eldar because they enjoy being the villain or because they like look of Dark Eldar.
*Does cost matter?
While models are priced according to their utility in the game (at least that is the theory) certain forces are more expensive than others. Usually anything included in the latest starter box is cheaper in reality than theory would suggest
*Does being able to defeat all opponents matter?
Certain factions seem to have great difficulty against other factions. These mismatches usually change over time and from edition to edition (sometimes a new Codex renders a certain force much more powerful than before).
*Will you consider proxy models?
There are many manufacturers who produce models that will fit with GW models in terms of style and size. While these models are not allowed in GW stores or GW sponsored tournaments that may not matter to you.
A few examples:
These Trollforged make good Imperial Guard proxies:
link
and these will work well if you have an Inquisitor and can fill a few other roles as well:
link
If you are interested in Chaos these figures Ultraforge will likely be of use:
link
Some manufacturers, such as Scibor, can provide virtually anything you might want for several 40K armies:
sciborminiatures.com
All-in-all you will find you have many more choices to make, from rules to models, than was the case 20 years ago.