For d20
Modern, excluding the core rule book -
Most used: d20
Weapons Locker
Least used: Menace Manual
Now here's the thing: If you asked my opinion on these two books, I would tell you that
Menace Manual is very cool while
Weapons Locker isn't all that great - yet I use
WL quite often in running my Modern games while the monsters and organizations of
MM are just interesting reading to me. Until now, I never really noticed that...
I did not consider TGMs'
Modern Magic - it is "never used," not "least used" (but that's no reflection on the quality, rather I haven't had need of it yet). I actually used a set of yacht plans and stats for a helicopter from
Menace Manual, so it is in the category of material that has appeared in at least one of my games.