The original series OST is very nostalgic to me, I especially loved the sad pieces (like Inori/Prayer and Cloudy Weather, then Tears), the music that plays when they part ways with a friendly trainer they meet (A Meeting and a Parting) made the show so much more heartwarming.
I was sad BW! began a tradition of no longer using old pieces, but there are (as always, for the Japanese version) beautiful pieces there too.
Chiisaki Mono (one of the only old pieces used again in BW! for the Jirachi episode) is one I love a lot, even the English version maintains it as May and Max's lullaby... "I sang, without understanding the meaning, a love song" / "that summer I was afraid of swimming" it captures nostalgia of childhood memories so well.
There are beautiful sad pieces in BW!, but as they were never released, we don't even have names for them... (it's used ironically in this scene, but) the Koaruhie/Ducklett episode when Satoshi/Ash tries to explain to them how important the sunglasses are, so they'll give it back, there's another heartwarming piece that plays early in the episode with Yabukuron when Ash speaks with the kids in their secret base/treehouse...
Also, the music that plays when Shootie/Trip leaves the Champion (Adeku/Alder's debut episode!), disappointed, and when Satoshi/Ash asks him where he'll go, he says he'll follow the path he believes is right... ah! I think it's the start of his extended theme!!
There's also the Nuvema town theme, it's very sweet and feels like an early morning. a Professor Juniper's lab theme too!
I also love a lot of the ending songs!
Soko ni Sora ga Aru Kara (Because the Sky is There) and Smile from Advanced Generation were particularly beautiful and comforting, with beautiful lyrics and messages...
Te wo Tsunagou (Let's Join Hands) and (Fanfare of the Heart) have similarly comforting, but also encouraging airs to them, and sweet lyrics... Fanfare of the Heart particularly fits the character arcs ("say goodbye to the yesterday when you'd run away from being hurt, even though the wind is still strong" fits Iris so well, since she ran away from the bullying and isolation of the academy, but still has trouble opening up to her Pokemon and human friends, "whoever said that being by yourself is better than being with everyone?" can work for Shootie/Trip, or even early more arrogant Dent/Cilan... and victory and defeat not being everything is so Ash!).
Mune ni Lalala had such a gentle end-of-summer nostalgia to it, a little lonely, it fit the end of DP.
Of the openings, I loved Battle Frontier, Yajirushi ni Natte (Become an Arrow! ... the second version for Episode N) and Summerly Slope.
