Hmmmm, this needs a poll, methinks!
1. Deflag
PC. Not much PC in the game, so an spell (instant kill) PC is so valuable to MANY MANY decks.
2. Golem
Let it live a few turns and it becomes unkillable (save Freeze + Shockwave, but who does that outside of Neptune?). Immo-golem is always good rush-fun.
3. Fireshield
I love Firestall, which is totally incomplete and fail without these.