Normal dragon chests are level 5s. Greaters might drop level 6s, but not about to go after a paragon Greater (the normal paragons are hard enough).
Not all named creatures are level 6; Lurg only has a level 4 chest. The Titan, AW, Balron PE & BE chests are level 6 because they are both normally level 5, AND just happen to be the spawn of a level 6 map (I think the devs looked specifically at what would have a level 6 map as a paragon, and chose the map spawn from that small pool).
Named Creature Maps known:
Lurg: Level 4
Pyre: gives level 5 maps, reported to give chests; so the chest would be level 6 (not personally got one yet, from the difficulties involved with getting past Rend)
Miasma: Level 6
Rend: Level 6
Putrifier: Level 6 (it's a named Balron - what do you expect?)
Abscess: Level 6
(I have chests for Putrifier, Rend and Abscess, by the way)
Named Creatures with unknown Map/chest status:
Flurry (very rare "trap" spawn in Labyrinth; named air or snow ele, unsure which, and so rarely encountered that only recently have I even found on Wiki sites, and had no entry as yet on Stratics)
Tempest (similarly rare named air ele or energy vortex in labyrinth).
Szavetra (named Succubus in Sanctuary)
Moug-guur (named ettin in Sanctuary)
Chikkaha the Toothed (named rat mage in Sanctuary)
All the creatures that spawn in The Prism of Light (most, if not all, might have maps, but chests are unlikely).
Not seen any reports of chests from any of the named Weald or Bedlam spawn, or the equivalent leader types from the Citadel (that the keys come from).
The wisp chests are level 5 (and a pain in the arse to open. I broke 30 picks on one chest, opening about 20 for a friend - 2 others opened first try. %$@#$@#$ random number generator).
