it's not an easy question for me!
i'll highlight two from undertale: first, it's pretty cool how waterfall is kind of a "leitmotif bridge" between ruins and another medium / core. too bad snowy isn't connected in the same way
more notably: the way don't give up uses the variation of the ruins leitmotif from an ending / undyne's neutral death theme is pretty amazing. with pretty much the exact same melody and rhythm, it manages to go from depressing and emotional to inspiring... and emotional.
for deltarune, i'll highlight two from chapter 1 and one from chapter 2. i like how field of hopes and dreams, as the very first "proper" area theme, heavily emphasizes the don't forget leitmotif, using it in a cheerful, upbeat way.
as for one of the very last songs in chapter one, the way chaos king just blasts you with the legend section C in a refreshing burst of determination and hope that contrasts nicely with the rest of the theme... yeah even in chapter 1, despite lacking the various improvements of chapter 2, toby fox absolutely nailed it with everything music-wise.
and as for chapter 2, the way berdly's theme is incorporated into attack of the killer queen is both surprisingly subtle (especially since it actually takes up more of the song than queen's own leitmotif(!)) and, just, pleasant to listen to. in particular, i love the way they changed around the rhythm for section B of berdly's theme.