OTT remains my personal favourite (respecting that it's personal preference), though others are extremely useful as well. It covers a pretty wide area, has a clear layout with very easy to interpret route settings on most maps. The obvious weak point is the link to schedules, which is very haphazard, but usually resolved by clicking on a nearby station instead of the train.
The area coverage on Railcam is second to none, and headcodes usually display properly and click through to schedules correctly, with allocations if entered, but for me the layout of some maps is too crowded and there is the annual subscription cost. There is also only limited route setting information shown currently.
Signal Maps has the potential to be the best, but at the moment only covers quite a limited area. The detail in what it covers is excellent though (on most maps the best available), with distant/repeater signals showing yellow/green, flashing aspects, berth occupation and level crossing status (which OTT only has on a couple of its maps). Given the choice I prefer a black background, and the colour codes for how punctual the train is and a link to RealTimeTrains is a big plus. My major 'gripe' with it is that the route setting information is more difficult to interpret than the crisp clear green lines on OTT. If tracks where routes were set showed up in green, or white with other lines in grey, that would probably be enough for me to consider it the best.
Traksy is also perfectly reasonable, though that's the one I have the most problems with on a mobile device and the layout and colours isn't to my taste.
It's fantastic that these sites are available though for free - or at a modest price for Railcam, which obviously includes the live cams too. A huge thank you to all those who develop them for us to enjoy.
I wonder whether the recent widescale adoption of GPS based reports to back up automatic and TRUST/TOPS manual reports would make maps for areas where data feeds aren't available, a much more workable idea now?