I'll try to help you, but no study has been made to rate the safety of individual
flavors. Some flavorings might have ingredients that change to toxic substances when vaporized. That's a theory considered likely, but no studies prove it either way.
Since flavorings seem to present more of a likely hazard than either propylene glycol or vegetable glycerine, the presumption of safety would then be
tobacco flavors like virginia, flue-cured, Fargo, CML, cigar, French Pipe, etc. I find some too harsh to use -- particularly the cigar-Heavy Shag blend. I get some harshness from Fargo (flue-cured) as well.
Since I don't have breathing problems, my preferences are Janty RY4 and Cappucino. Neither has a
tobacco taste but they are excellent.