1.  "Mr. Kovac" - the obvious classic
2.  "Kovac" - Although some might find this disrespectful, there are actually a number of students who omit the title of a favorite teacher's name as a form of respect.
3.  "(Big) Papa Kovac" - In contrast to another teacher who was called "(Big) Mama _____."  Originated in what will probably be my favorite upperclassman AIS workshop, which they unofficially named "Math Prep."
4.  "DJ Kovac" - When I told my students that I have a Twitter, they went looking for it.  They were adamant that this was my handle, but it (obviously?) isn't.  Mine is @KovacMathHelp if you care.
5.  DJ Kovs - Pronounced "Coves", it's a shortened version of #4
6.  Mr. C - From my lower-level AIS workshop, they had a habit of calling people the wrong names on purpose.  Everyone ended up having an alias, and this one was mine.
7.  Kaptain Kovac - Just got this one a week or two ago by a student who I accidentally called Patrick, because he reminds me of Patrick Star from SpongeBob SquarePants.  But I haven't told him that part.  Found out, this week that more than one of my students refer to me regularly as "The Kaptain."  (Note the alliteration.)