I have eclectic tastes when it comes to buying trains - I have prewar, postwar, and all forms of modern era Lionel (MPC, LTI, LLC, and modern Vision/LEGACY), along with K-Line, MTH, Weaver, and Atlas. So, I have a little bit of everything, and they're all ones I'm very happy to have.
All that being said, the majority of stuff I hunt for nowadays is a mix of postwar and LTI era Lionel. Both eras' (especially the latter) stuff you can find for absolute bargain prices (compared to back in the late 90's). Plus a majority of the LTI locomotives, early LLC locomotives and MTH PS1 locomotives are scale and can be upgraded for inexpensive prices, all the while still spending less on a current LEGACY or PS3 version.