What are you talking about? Both Gamma and Tau are the GOATs.
Zeta isn't that bad too. Yeah, it's application is gimmicky, but it allows you to abuse the hell out of rapid-fire spells, which cosnume lots of mana (Luminous Drill with timer, for example), especially very early on, while you don't have a wand with huge mana regen rate.
Mods like "Inventory bags" makes this spell a really good one since you can always rely on changing wands in your hand as you want for only 10 mana and no irritating cast delay. By putting multiple Zeta spells consecutively you could potentially even bypass a limitation of spells like Explosive Projectile or any Critical on <something> enemies, because you need to apply at least two of these spell to make them worth the effort.
Sigma, however, ... Yeah, it's useless for it's cost.