The early to late 1970s, i.e. before thermals, as UK v Warsaw pact or WP v NATO. That's the era I
keep coming back to - being as I was a weekender in the Territorials back then may have some bearing on that
! Soviets have some competitive toys in this time-frame, especially later 70s.
South Africans v Cubans is also good in the same sort of time frame playing either side. It is historic as well. Batlocs tend to produce open deserts and savannah, good tank terrain but watch out for those Saggers!.
Iran v Iraq, when the Iraqis have those darned nasty T-72s is a good period for an LC as well. Another historic pairing and the war lasts a long time (if you want a decently long LC) and the batlocs are in the code for it all.
The various later India v Pakistan conflicts are also a good option for a battle, usually as the Pakistanis since they are behind the Indians in tank tech. '47 war is a bit too WW2-ish though. And again its historical.
The last 3 are all ones I play on occasion as they are reasonably balanced, or produce a challenge to the human player (e.g. with just 90mm Pattons as Pakistan in the 65 and 71 wars).
If nobody has thermals then you still have some ability to use traditional tactics, smoke and so on together with normal recce methods - no all-seeing "eyes of Sauron".
cheers
Andy