Ari Voukydis was my Level 1 teacher. That class was a blast, one of the best improv classes I've taken, and a lot of that came from Ari's enthusiasm and encouragement. I could tell that he really loves improv, was genuinely psyched for the class, and he never phoned it in or faked through it or was too harsh or too easy on us.
Back when I was signing up for Level 1, a performer suggested I go with Ari or Betsy Stover, and now I've got the privilege of performing with both of them. Crazy, huh?
PS: Looking at the list of who's currently teaching Level 1, everyone is good, but I'd also recommend Jeff Hiller. He lights up any room he's in, will make it a fun three hours and I've found him to be really spot-on about notes in a positive way.