I do not own Falklands or Modern War so I can not comment on them however, I do own the others and have played them only by PBEM. With me being a life-long fan of WWII, I would have to say that RV would be the "must have" title from your list.
As has been stated numerous times in the past in the forum boards, RV is arguable the best SB WWII game released to date and is also considered one of the top SB games found in the whole series.
If you have any interest in WWI, SB FWW can be a fun game to play. It comes with a ton of background info and covers the whole war 1914-1918. Just make sure you bring your own gas mask or you may find yourself being gassed by your opponent!
I had a hard time getting into SCW (Spanish Civil War) due to the very low range/"weak" weapons that are used. Out of a handful of scenarios I played, neither opponent had a machine gun or any armor and it felt as if most of the units had a pistol or a out-of-date rifle. Because of this, I feel it requires a different set of tactics than the ones you may find yourself normally using in the other SB titles, since it is very hard (at least for me it was) to offer suppressing fire in your attack.
Dien Bien Phu is a good title covering the early part of the Vietnam War. I have played some fun battles in that title. It was the last title released to cover the nam series and has been called, more than once, a good and underplayed game.
Good Luck on your decision. Let us know what you end up getting.