Aftershock is always going to have a special place in my heart for one simple reason: never have I felt so satisfied seeing a nicely trained team of alien-battering monster-stompers suddenly get handed modded weaponry. Sticking all the lovely extras on to your guns and then to actually see them in teh game environment AS WELL!
What can I say, it's like freakin' Christmas all over again, but with more death.
Aftermath is also a special place: It had been too long since we saw an isometric squad based strategy game and then along comes AM and they had taken all the best bits from XCOM APOC. and, well, polished that too!
To be fair, AL is a wonderful game and it provides a whole new perspective to the genre, but that's what they've done for all of the games - shown us another way. So if you're going back, go back and give them both a tumble and remember why this market needs MORE of the ISBSs - because there's NEVER ENOUGH OF THEM!