I keep hearing that this series is so damn good, but I have zero clue in which order they go..
Play it like this:
Muv-Luv(finish at least the Sumika and Meiya route to unlock Muv-Luv Unlimited)
Muv-Luv Unlimited(its packed with Muv-Luv)
Muv-Luv Alternative(All "routes" have the same ending, that's all I have to say)
If you want the chronological path:
Schwarzesmarken(No EN version yet but if you want to read Japanese you can buy hard copies on ebay iirc)
Muv-Luv & Muv-Luv Unlimited
Muv-Luv Alternative
Then you can read anything after that. Photonmelodies is basically continuing where you left off in Muv-Luv, Total Apocalypse & The Day After is for when you've read Alternative.
Also I missed some titles here that only the hardcore fans really know about this series. I hope you enjoyed Muv-Luv as I did. Good hunting