A warning. These aren't so much a review as more a "what's out there". That said, YMMV when it comes to rules (one of the great debates of miniature wargaming, hell, even Empire Napoleonic Rules has its fans, flow charts for cavalry charges and all).
So, without further ado...
Force on Force by Ambush Alley Games/Osprey (2011)
(A Disclaimer: I helped playtest these rules, as well as a good chunk of the supplements. I cannot say I can give an impartial review.)
Force on Force is a very solid set of rules if you're looking to run anything up to platoon level (which is what most Twilight 2000 fights are going to resemble, but not all mind you (Something like out of Ruins of Warsaw might require something a bit bigger, we'll have some suggestions out there later).
Force on Force has an asymmetrical turn system that, admittedly, takes some getting used to (I am still not 100% sure I get it right all the time) and to be honest, if you don't have enough on the table in terms of terrain, both sides are going to sit back and shoot each other to pieces, (but this is a flaw of MANY modern wargames, not just Force on Force).
Force on Force is very, very adaptable as a set of rules, and with the die types (anywhere from D4 to D12) being the modifiers rather than the usual litany of DRM, this is on the whole, an easy set of rules to pick up.
I would state these are one set of rules you could not go wrong with, you could use the low supply rules for all sides to reflect the nature of Twilight 2000, and adapt the Fog of War Deck from Cold War Gone Hot to give the game a more "Twilight 2000" type feel.
There is a 2nd edition being worked on, with an indeterminate release date.
Where to get it: Amazon.com (it's pricy through 3rd party sellers)
Ambushalley.net (PDF Only)
Cold War: 1983 by Wessex Games (2005, PDF version 2011)
These rules are a no-nonsense, no frills product that is geared mainly towards the 28mm market, but you can easily use these rules with 20mm figures with little to no adjustment. The rules are D6 based and revolve around a "bottle" system which synthesizes a variety of soft factors like morale, training, friction and the like. Failures against one's bottle can decline the score to where the morale of the individual figure collapses. Bottle tests factor into everything, including movement, so there is a big factor of unpredictability in the rules.
There are optional rules for skill levels which add a role playing element into the game, and vehicles are handled very simply.
In short, these would make a good set of rules to use to run combats for the RPG, but anything bigger than a couple squads a side might bog the game down.
A bonus feature of the rules? It mentions Twilight:2000 and the Zone novel series by name.
A 2nd edition is supposedly being worked on, but there has been no word for a while.
Where to get it: Wargame Vault (PDF only)
Firefight: Modern Skirmish Rules by Tabletop Games (1985)
These rules are definitely a product of their time. I played them a couple of times, and while they have a following in the UK, they were a bit clunky when I played them. In one game, a Sagger shot on a T-34/85 (it was a Yugoslav Civil War game) took at least 3-4 charts and much leafing through the rules to resolve.
That said, they were comprehensive, and the charts, while hard to read, had stats for every small arm and infantry weapon you could possibly need. The rules aren't in my opinion, going to handle anything more than a few squads a side well, but then again, it's not the stated purpose of the rules to do so. While it may not be my cup of tea, the crunchy nature of the rules do appeal to some, and if that's your bag, then you can't go far wrong by grabbing this set of rules.
Where to get it: ebay, Amazon (it's been out of print for a while, and is hard to find).
I owned a copy of these rules when I was a teenager and I used them for everything from WWII to Science Fiction, and they did the job well. The clarity of the rules was a bit to be desired, but the rules themselves worked well, and was one of the first rule sets I remember being more about the "soft factors".
Wounded cause more morale penalties than dead figures do (the screaming of the wounded having something to do with it, according to the designer) and leader figures are very prominent in these rules.
While the rules are set to be an insurgents vs. government set, I have run them with two symmetrical armies in contact with little if any trouble.
The rules themselves are long out of print, but if you can get a copy, you cannot go far wrong IMO for a Twilight:2000 milleu.
Where to get it: You're really going to have to look hard for this gem. I remember getting my copy second hand in the 90s at an HMGS con.
Maxim to Minimi by Jason R. Weiser (1994)
These rules are meant to be the ultimate in simple, and they live up to it (I should know, I wrote them). They really are bare bones, and to be honest, could use a lot of work. The rules are mostly D100 percentage based, with a D8 for wind/arty drift.
The rules cover wounds levels, and would probably do well with a lot of counters..which would not do much for table aesthetics. Movement is unpredictable, and is one of the things I like about the game.
The system is simple IGUGO, and there really isn't much more to say than that, as it's not my first recommendation for a set of rules. That said, they are free.
Where to get it: http://www-solar.mcs.st-and.ac.uk/~aaron/MtoM.html
No End in Sight by Nordic Weasel Games (2014)
Ivan Sorensen of Nordic Weasel cannot be called anything less than prolific. And as a bonus, he writes some pretty good rules. I have not played No End in Sight (have a copy, but haven't done much modern skirmish lately, part of the reason for this blog). The rules are in their 2nd Edition now, and in short, read very well.
Everything is handled simply, and the rules are based around an idea of units accumulating shock, and through that, becoming ever harder to control. Most fire is pinning fire, with assaults being the decisive measure (as it should be IMO). Three campaign systems come with the game, and more can be generated very easily.
While the rules say they are meant for 15mm, it wouldn't take much to convert them to 20mm.
In short, these are a very go to set of rules, for a very economical price.
Where to get it: wargamesvault.com
Living on the Frontline by Partizan Press (2016)
Living on the Frontline is the first in a series of supplements for the Winter of '79 milleu. I own a copy, mainly because the background is cool as heck, and while I haven't taken that close a look at the rules, I will state that they look solid for the subject matter.
The main thing, is that you're going to have a lot of work converting in the Americans, Germans, Soviets and everybody else you're going to need for Twilight:2000, that said? It shouldn't be too difficult and considering the rules are already geared towards the "chaos in the streets" one finds in Twilight:2000, I don't see a reason why these rules would not work?
Where to get it: Living on the Frontline Page
Black Ops by Osprey Wargames (2015)
Black Ops is another set I own, but have not played. It's more geared towards the kind of Special Operations-type stuff you see in today's news. It has a lot of nice bells and whistles, but it would take a lot of work to convert it for Twilight:2000. That said, to me it looks doable. The main thing I think would be adding AFV.
But that said, the rules are very fast, clean and have a mission generator and campaign system. These rules would work very well if you're playing a campaign of a bunch of PCs raiding, oh say, the New American enclave in Florida for Civgov or Milgov?
Where to get it: Osprey Publishing Page
So, Part 1 is done. Part 2 will cover rules meant for larger engagements, there are not a lot of modern sets for this, and most are meant for 6mm, but with a little work, most of them can be easily converted to other scales.