!wa is the quickest !bang for Wolfram|Alpha*. For anyone not using DDG yet, you can see all the !bangs here. I only use a few as well, but it's extremely powerful to be able to search other sites directly from any DDG bar.

Recently I tend to use
!w (Wikipedia)
!v (DDG's video search)
!wa (WolframAlpha)
!w3s (w3schools)

I still use Google's stuff, but find it easier to use DDG's search bar, since you can just type to search & get to any of them directly (no mouse required).
!yt (for something I know is on YouTube)
!m (Google Maps, since !osm OpenStreetMap doesn't work well where I live)
!n (Google news)
!g (Google. I find plain DDG to be better than Google for most queries, but if I need the answer to a specific question that will be on an obscure forum somewhere, Google is still better at finding those).

*(!wra, !wolfram, !alpha, !walpha, !wolframalpha also search WolframaAlpha)