I said 2nd level. From my perspective as DM, I must say I often have to challenge the PCs using lower level spells than the PCs themselves have access to. 2nd level does this nicely, I rarely feel the need to use 3rd level spells instead and consequently upping the EL quite a bit.
Also, One of the PCs is a 9th level Conjurer right now. He still uses a lot of 2nd level spells, since they don't run out of usefulness: Alter Self, Invisibility, Glitterdust, and Web being his staples. He often says "why use a 4th or 5th level slot when a 2nd level one will do the job nicely".