Those pronunciations are:
(1) “ser-shee-or-RAHR-ee,” (Stevens, Thomas, Alito) (although Thomas has his own "sertz-ee" variation);
(2) "ser-shee-or-RARE-eye," (Roberts, Scalia, Breyer);
(3) "ser-shee-or-RARE-ee," (Rehnquist, O'Connor, Souter);
(4) “ser-shee-or-ARR-eye,” (Kennedy);
(5) “ser-shee-ARR-ee,” (Sotomayor); and
(6) "Cert." (or, phonetically, "sert") (Ginsburg, Kagan) (both of whom have simply chosen to avoid using the term in full).
The lists covers most of the variations that I have heard, although one of my professors at the University of Iowa taught me to pronounce "certiorari" as "ser-shee-AIR-ee." While I was never criticized for using this unique pronunciation, I became more self-conscious about how I used the term once I came to law school and found the word to be a bit more commonplace.
Now, when I find myself in a situation where I must say "certiorari" aloud, I take the third approach that Rehnquist, O'Connor, and Souter adopted. Unfortunately, all the justices who have used this pronunciation have left the court, and Duane points out that the pronunciation disappeared from Garner's Dictionary of Legal Usage following Souter's retirement.
During the first year of law school, one of my professors told my classmates and me that uncommon words or pronunciations should be avoided when possible, even if they are correct. Using uncommon, but correct, terms may lead to an unfavorable judgment by the listener, even if that listener turns out to be mistaken. With this lesson in mind, it seems that the ideal approach to take when pronouncing "certiorari," is to take Ginsburg and Kagan's approach and simply avoid saying the full term.