Depending what you mean by "regularly", my answer would be all of them.
Thing is, I pick specific roles for specific ships and I have 3 separate "fleets" throughout the bubble so I don't have to transfer ships when I move from, say, fed space to imp space or to the Pleiades.
As a result of that, I've got 3 Annies which are all almost identical heavy-multirole ships - and they all get used regularly when I'm in that area.
I've also got 3 almost identical DBX runabouts, 3 AspX passenger ships, 3 AspX miners, 3 combat Corvettes, and 3 light Multirole Kraits which are all available for use whenever required.
Ironically, the ships which see least use are probably the ones I've spent most effort on; the "vanity projects".
I've spent ages fettling my Vulture so it's a pocket battleship and building my "Project Blackbird" Viper, iEagle and iCourier and none of them really see a lot of use.
To be fair, I did use my Viper regularly for doing Palin's "thargoid tat" missions but since FDev nerfed that it's not worth the effort.
Also, my iCourier has gradually gained weight (in the form of useful modules) to become a sporty long-range runabout but I usually use a DBX instead.
