Most of the rules discussed here are for battles larger than company size, and often one stand will equal a platoon in most of these rules. As Twilight:2000 as a background doesn't exactly lend itself well to "exact" units, you may need to do some tweaking to make these rules work, but abstraction (especially when it comes to doing fights from modules like Ruins of Warsaw) will probably work better than not.
So..without further ado...
Tractics/Tractics II by Guideon Games, then TSR (1975/1977)
This old game is to put it mildly, a relic. Even in the 70s, it was a clunky piece of rules writing, with rules for just about every bit of armor/anti armor interaction one could think of. I owned a copy in the 90s and yes, played it. It took hours to get through a couple of turns. But, I liked the D20 system, it made percentages easier, and the morale system was, in my mind interesting.
I guess my biggest issue is that the game was frozen in time, for WWII, it would probably still work (and did in an update called Battalions in Crisis), but for moderns (and Twilight: 2000) would probably need a big facelift, heck in the game, the recoilless rifle and spotting rifles are still the bee's knees.
But, if you're willing to put in the time, and find crunchy wargames to your taste? Then this is the game for you.
Where to get it: Honestly? If you find a copy? Let me know, I would love to know where it's still for sale?
Cold War Commander by Peter Andrew Jones (2012)
Cold War Commander is one of the newer generation of fast play rules that caught on in Britain, and soon came here in the guise of games like Flames of War. It's point driven, and says it can play up to 20mm with no rebasing. Now that said? I'd really stick to 6mm with this set. The things I wasn't nuts about the few times I played it? I had to do points lists instead of actual units, the command and control system was a bit..odd (very reminicent of Black Powder, now that I think about it), and to be honest, the points themselves seemed a bit arbitrary. I prefer Fistful of Tows. You'd have to add some things for Twilight 2000, but it would not be impossible.
That said, it does play ungodly fast. Games do get done in a few hours and it could handle in 6mm the larger fights sometimes seen in T2K pretty easily. The rules themselves are a bit pricy..but the production values do make it worth it. The rules illustrations alone are very well done.
You could do a lot worse than buy this set.
Where to get it: Lulu has a monochrome perfect bound version for sale. The full color copies can be found for sale on ebay and Amazon or The Miniatures Page occasionally.
Modern Spearhead (2000)
Disclaimer: I am not a big Spearhead fan in general, and while I have only played MSH once, it did suffer many of the same problems it's WWII ancestor did.
Modern Spearhead is much like it's WWII ancestor. It's a game very dependent on order writing (thankfully drawn orders on a map, but I have often seen that lead to disputes on the intent on said orders).
The game fixes some of the issues it's WWII counterpart had, but then creates new ones. My really big issue was the way it handles infantry, where they are basically tied to the carrier (to the extent that you have to base the two together. I submit while infantry squads have shrunk over the years..this is taking it a bit too far.)
The command and control rules might not work very well for T2K, on the basis of the fact they are meant for armies that still have a semblance of modern command and control. I suppose you could use the rules for the conventional phase of the war, and more power to you if you like them, but for the broken backed phase that lasts until 2000? There are better rules sets out there in my opinion.
Sorry Spearhead fans, it's just not doing it for me.
Where to get it: On Military Matters, Ebay, The Miniatures Page occasionally.
Rapid Fire by Rapid Fire Publications (1994)
Rapid Fire is the granddaddy of the "fast play, large combat" rules we have seen coming out of the UK since the 1990s. While the rules are geared for WWII, there are quite a few free online upgrades to bring the rules into the modern era.
While I own a copy, I have only played them once, at Historicon one year. They were a solid set, and while they didn't knock my socks off, they did what they set out to do very well. It's a shame the rules aren't as popular in the US for some reason (One gamer decrying my decision to buy a copy, telling me Rapid Fire was referred to as "Rabbit Farts". Why, I really do not know?)
The Guild has used modified Rapid Fire for their "Big Games" for years, and I have not heard any real hiccups with them, so if that isn't a ringing endorsement, what is?
Are there abstractions in the rules? Yes, but they are there to make the game work, and in that, they work admirably,
These would be a good set of rules to start with for some of the larger battles in Twilight:2000.
While it is difficult to get a copy of the current 2nd Edition (A third is being worked on), it is still out there to be had in PDF.
Where to get it: Wargamesvault.net (PDF Only)
Command Decision - Test of Battle by Test of Battle Games (2007)
Command Decision is a venerable set of rules, having been through three revisions as of this writing (the first edition being released by GDW in 1986). It's a solid set of rules that accomplishes what it sets out to do, but I personally think it runs a bit slower than more modern designs. While the current edition is set up for WWII, again, you can find stats and information for modern conflicts online, or if you happen to get lucky, the modern version of Command Decision, Combined Arms, at a convention or online. It should run most of the larger battles one finds some of the Twilight:2000 modules, or of your own creation, just fine.
A mention here also, if you happen to get a copy of Challenge 25, either in dead tree, or at Wargames Vault? Pull out the "Twilight Miniatures Rules - Draft format" and take a close look at it and Command Decision. There are a LOT of similarities. Was this a proto-Command Decision? I dunno.
Where to get it: Test of Battle Games Website
Fistful of Tows 3 by a Fistful of Games (2011)
Fistful of Tows 3 to me, at least, is the best set of operational level rules out there. It runs simple, clean and with D6 only. It's common sense, it allows you to add new equipment and infantry units to the game pretty easily (though calculating PV for equipment is a bit of math slog). It should handle converting anything you find in Twilight:2000 with little trouble.
The size of the book should not intimidate you. Most of it is army lists and equipment stats..and boy are there equipment stats. In short, this is the motherlode of information. Mr. Beard and Mr. Minson back up their assertions with fact that is cogently written (it helps that Mr. Beard is an attorney), and all in all, the rules are some of the best I have seen for this level of warfare.
My main issue? Like most 1:5 scale rules, you really are better off going 6mm with them. 15 and 20mm (especially at the WWII ranges stated in the book) just don't run well. You can spend more than a few turns advancing to get into range. Now, for Twilight:2000, that should not be much of a factor, but keep that in mind when using these rules and mixing the technology.
While the pricetag is a bit on the high side, there is a 32 page free preview so you can "try before you buy". In short, a great idea more rules publishers should follow.
In short, I heartily endorse these rules.
Where to get them: Wargamesvault.com (PDF Only)