[Sledgehammer427]

It seriously depends on what you are looking for and how much you are willing to pay.

For WWII, you have a choice between Steel Fury (SF)
and T-34 Vs. Tiger.

T-34 vs Tiger, isn't very replayable, it comes with 12 missions, 6 for each side. they are all scripted, and it's only real saving graces are Zeewolf's mods and it's graphics.
Zeewolf's mods are costly. even though the games in the bargain bin, if you can find a copy, Zee's mod will set you back 60 dollars for the first year. but thats access to all of the mods he releases and yes, it costs less to renew your subscription. you can check the Kharkov thread sticked at the top to follow development.

the other option for WWII is Steel Fury, this is more playable in that the maps are large and it's quite realistic. all of the bugs have been worked out for the most part and theres a huge selection of mods. the current one is Steel Panzer 1.5 (beta) which adds a whole lot of playable tanks with more planned to be added before the official release, that is, the official release on the sukhoi forums. it's hard to find as well, but should be cheap and is well worth the price. There is a resources thread stickied at the top that I update as soon as new mods or information comes out

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now, the devs that put out steel fury are coming out with a cold war-era tanksim called Steel Armor - Blaze of War, stickied at the top of the page.
Still in development though

Now, Skybird can come her and clarify a little more on this, but the benchmark tanksim, which all other tanksims are thankfully not judged but aspire to be.
Steel Beasts Pro Personal edition. it's modern but it's a pared down full-on military siluation, used by...militaries.
The only issue would be that it costs 125 dollars. it's apparently the best tactical simulation money can buy. eSim games knew there wouldn't be a big market, so that's why it's exorbitantly priced compared to many other computer games.

I own both TvsT and Steel Fury, Steel Fury is heavily modded but TvsT is mostly stock. I enjoy both, and as I said, Skybird knows more about Steel Beasts so I hope he can come along and offer more information.