If it was really sweet it may have been an Ujukitsu. They are usually handgrenade looking. Another is the sulcata.

chugga chugga chugga.... :-)

Septoria rot Identification tip: Infected fruit develop small, light tan to reddish brown pits in their rind. Bacterial blast also causes small black spots on rinds; copper fungicide injury looks very similar, but with phytotoxicity there are no brown to black Septoria fungal fruiting bodies growing in the pits (as shown here). Septoria rot pitting is usually relatively shallow and does not extend deeper than the oil-bearing tissue. Anthracnose Identification tip: Brown to reddish green, discolored streaks on the rind called tearstaining are symptomatic of anthracnose (shown here). Septoria spot and occasionally other fungal diseases cause similar discoloring. Bacterial blast Identification tip: Infection results in small black spots on the fruit. Also called Citrus blast or black pit on lemons, dead leaves and twigs are often present when the cause was infection by this bacterium.

I haven't done a lot of zest tasting across other varieties but I am always impressed with the intense lemony aromas from the Ponderosa. It really doesn't fit your non-bitter requirement though. I think some do consider it a citron. It is bitter by itself but does work very good where zest is called for.