#5 - Wall of Fog (4th Edition, 371/500)
Attacks on your realms are bad in Spellfire. They lead to realms being razed, and spoils being awarded to players other than you. It's even worse in a format like TAV, which promotes fast play and tons of instant-kill cards. A good strategy, then, is to avoid being attacked at all, especially in critical situations (like when you are within spitting distance of a sixth unrazed realm). Fences are cards that prevent attacks, therefore they are your friends. This one is a wizard spell, which means it has a drawback - it's easy to counter. It also stops you from attacking other players, which can be a problem if getting a spoils is a priority for you. Still, it's a decent card to play if you're close to winning and would rather not have to engage in combat.
#4 - Solid Fog (4th Edition, 133/500)
#2 - Forbiddance (Artifacts chase, 11/20)
#1 - Bronze Dragons (Dragonlance, 97/100)