Consdidering what the OP asks:
1) 6mm
2) brigade size forces
I also admit that in 6mm I think larger games with platoon scale stands are my preference...
I will present my extremely well considered suggestions, that are, being based on my infinite and supreme wisdom, the final word in superior rule sets
(to be clear it was just a silly joke
)
1) Spearhead: I found them the best rules for brigade/divisions. They are not overly complex, there is differentiation between individual platoons, the different tanks behave differently, and, on the table, battalions and companies work as battalions and companies. Some people dislike it because they tell me, the rules do not give players sufficient freedom. I always counteract that it is because those players become fixated with single stands rather than formation.
2) Command Decision: same level, more detail. I like the armor system quite a lot. It gives player a bit more freedom on the single stand front,but the order system still forces players into looking at formation (of course some players end up thinking by stands... and complain)
The big difference between CD and SH is command. In SH it is more effect based (you plan, your 6mm staff and battalion commanders 'implement' it at the best of their abilities, for some armies it is not a big assurance...). In CD because the system is more process based you have to allocate orders and similar think. You could appreciate the work of your 6mm staff and why at times in SH they do not do what you want from them. They are two rules very close in my top ten...
3) Battlefront WW2. Works quite well in 6mm, work better with smaller brigade (basic maneuver element is the company), but suits smaller brigade engagement. There is emphasis on command (maneuver roll) but it lacks the emphasis on planning.
4) Fistful of Tow (despite the name it has organizations and stats for WW2): lower end of the spectrum for several reasons. It has very little planning and C3I, a bit too many rolls for some of my tastes, but really good hardware based system. Quality is still important. I Like it when I want more hardware focused games. I think it is better for armored engagement than Infantry ones, so combined arms engagements works much better when tanks are available aplenty.
These are my recommendations.
Assorted thoughts:
Like IASBM, works well in 6mm, excellent infantry system, bad (if not awful) armor penetration. I am simply using armor penetration from Battlegroup until I can figure my own system that I can then submit to Richard for one of the specials...
While I am a fan of Battlegroup, I do not think it is a good 6mm ruleset. Infantry casualties are still tracked singly.
FoW... I agree that it works in 6mm. Actually I got the impression it is a 6mm game sold for 15mm... I like the new Anti Tank system for Team Yankee (but I consider the bail out results more a suppression that actually people jumping in and out...). While I am not a fan of heavy guns on the table on a company level game... I think their ranging system works and it is a good way to represents spotting rounds and corrections followed by FFE. Yet, I think that, its point system is crap.
Not Recommended:
Blitzkrieg commander: being a big fan, playtested the original CWC (my name is in the credits!), but completely turned off by the firing system and artillery. Artillery is the idiotic deviation dice system (I have replaced it in Battlegroup, I think it is a mechanic we keep using more by habit than by design, certainly not by research). Firing is outwardly nice, I like it for infantry... but when you use it for tanks... it fails. Basic roll to save system, with too little differentiation in vehicles. I am also worried by a system that claims to be able to work as one stand is a squad and one stand is a platoon at the same time. Basic verdict (after having played for years): nice idea, but then you realize that it does not work better than Spearhead, with less detail, and possibly slower. It was nice in 15mm, but for 6mm there are better alternatives.
Honorable recommendation:
now this does not fit on the OP specs, but everyone recommend lower scale rules... Jim Day's Panzer. Extremely good armor combat. Boardgame based but designed with miniatures also in mind. Infantry is covered, and works. The command system is interesting for the scale (1 to 1 for vehicles, squads with special teams for the infantry). The rules are modular so you can add the detail you wants to the basic skeleton. It covers WW2 to Modern. Extremely well playtested (it was published originally by yaquinto!). GMT is publishing the game version now (with maps, very nice counters and so on), Strike Net publish the only miniature version, available from Wargame Vault. The vehicles have their cards with penetration and armor ratings (yes you can impact different areas of the target!).
This is my round down of games I played.