The ending scenes on fr/lg are pretty cool. I played Sapphire, then LG, Then Emerald.. It's looking like I'm going to have to get D/P soon too. =\
If you have a ds, go for Diamond/Pearl.
If you haven't, and you have real-life friends, who play emerald, I'd get that (because of the multi-battle in battle frontier.)
I think LG was more of a fun game, being able to re-live some of my former childhood, on a more state-of-the-art gaming platform, with some actual color xP