If we are judging by quantity it looks like this:
1) LTI era 6464 remakes. I have almost all of them including club cars
2) MTH RailKing cylindrical hoppers in Canadian liveries. At last count I have 35 of these.
3) Lionel Post War small milk cars both operating and non operating. I have one of most variations including the first one with the brass mechanism.
The cars I operate most are MTH Premier 18" passenger cars. I have these in many road names and just love the way they look snaking through the curves.
As usual, judging a "favorite" is not as easy as it sounds.
Rolland