It's hard to call. Either Magical Dimension or Magician's Circle. Possibly Silent MagicianLV8, though that's till I get Dark Magician Of Chaos.
MD = 2 for 1. I tribute a Spellcaster, get a usually better Spellcaster, and destroy an opponent's monster. Can even be done on my opponent's turn

. AND if done on my turn, I can still Normal Summon.
MC = cheap card. Means by my first turn I can have 2 Spellcasters to attack with and get a decent first strike in. Admittedly the fact it affects both players could be a problem, but as I'm the only player in my area running Spellcasters, it's a no risk situation
SMLV8: Immunity to my opponent's Spell cards and 3500ATK. Yeah, pretty decent.
Admittedly Dark Magician Of Chaos, when I eventually get him, will replace these.