The must have mods are 3. As Blacklight already mentioned there is lwami which improves the gameplay drastically over stock dw. Unfortunately it doesn't add any new playable units. Alfa Tau 3 and Reinforce Alert mods on the other hand add a lot of new playble units, subs and surface units even some air units. AT3 you can download from subguru's website, RA instead hasn't had a general release but it is getting near completion.
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