Since we're talking transformer upgrades from stock LC, I'm partial to the Postwar Lionel ZW (not to be confused with the ZW controller, aka ZW-C, which looks very similar, but in reality very different). If you go with a PW ZW, external over current protection is highly recommended either from a fast acting breaker ~$5-10 or a 135/180W Legacy Powermaster.
Postwar ZW features: 275Watts* ; 4 simultaneous and independent 6-20VAC variable pure sine wave outputs. *actual output is ~200-220W
Ones that have been professionally refurbished (replacing the original whistle rectifier with a modern one) can be found for about $130.
I have two PW ZWs and really like the flexibility they offer in powering independent tracks for operating conventional , LionChief, and TMCC/Legacy locos.
I also have ZW outputs dedicated to: 10V for Uncouplers, brightness control on scenery Lights, speed control of conveyor belts, and 16V to FasTrack remote, Command, etc switches.
A word of caution when choosing a suitable transformer to use with a Legacy Powermaster. According the the PM Owner's Manual "Transformers with a chopped sine wave output such as the Lionel CW-80, MTH Z-750, Z-1000, and MRC DualPower027 or similar transformers will result in unpredictable operation."
