Which primary tooth generally erupts last?
Textbook chronologies indicate that the maxillary primary second molar is most commonly the last  to erupt:

24 months (Woelfel and Scheid, 2002)
20 to 30 months (Kraus et al., 1969)
25 to 33 months (Ash and Nelson, 2003)
Maxillary second molar.
Mandibular second molar.
Maxillary canine.
Mandibular canine.