We (kids and me) like D&RGW and WP&Y.
D&RGW allows us to recreate the old steam trains, we like that ancient era because trains are shorter (loco + 3-5 wagons), colorfull (4-Bachmann 4-4-0, BumbleBee...) and easy to combine (you can mix boxcars, flatcars, combined, caboose, and different types of locos...), so fun is guaranteed.
WP&Y is quite different. I have almost completed two trains (one Bachmann based and one LGB based), got also one Bachmann 2-8-0 Consolidation, and now looking for (with patient) to get one of those LGB Mikado WPY nr 73.