As someone who used to work in the video game industry I'll give my 5c quickly that hopefully people can use as a good judge as to which to go for.
Rail Simulator is the best for graphical appearence, and with the new updates behaves realistically enough, but lacks new content at the moment, with around 4 loco add-ons including Class 66 and 158. Needs a relatively powerful computer.
MSTS has fairly poor graphics by todays standards but not the worst. It does however behave fairly realistically although, like Rail Simulator, it does not provide things such as TPWS so can lack realism. It does have almost every route, locomotive and stock you could ever dream of available for it by now though thanks to sites such as UKTrainSim (charge a small fee to download) or elseware for free.
The other simulators in general tend to be the worst graphically and have fair to moderate amounts of add-ons available (but not as many as MSTS) - however they generally provide the most realistic and difficult driving simulator, so IMO they cover a neiche market. I've not played every one though so I can't say that for certain of all of them and as such this is a bit of a blanket statemnet.
Either way my sim of choice right now is Rail Simulator.
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