Best
For my 360 I'd say:
PES 2008 - Far better than PES6 on 360, still find it addictive and really enjoyed the few times I've played it online. Has it's problems but still one of my favourite games this year.
Virtual Tennis 3 - Really enjoyed this aswell and found myself playing it for ages. Not the most difficult or complex games but addictive and annoying in equal measures.
For DS:
Pokemon Pearl - I like it as it doesn't stray far from the original and it made me remember playing red and blue when I was in primary.
Zelda - Still getting into it but I really like it
Bearing in mind I've only had the DS a few weeks, those are my choices.
Worst
Crackdown - What a letdown, hyped up as the GTA of 360, it was pish. Traded it in 2 weeks after purchasing it.