First things first I want to be clear that this isn't a question as to which of the options is your favourite manufacturer, but rather which one is arguably the best in terms of things such as reliability, efficiency, longevity and other engineering and technical factors. Now I myself cannot personally give a solid answer but I would argue that Alstom are good contenders because of designs such as the Pendolino which in my opinion has aged very well for rolling stock whose oldest units are reaching twenty years old. The design itself may be outdated and no longer meets crashworthiness and other modern technical standards, but it seems to still work as efficiently as ever. By contrast I would argue that CAF, while their designs may be aesthetically pleasing (I'm particularly fond of the 195 and 331) it's hard to deny that there are systematic design flaws in some of their stock, not just in the UK, but worldwide. Cracks in the bodywork, bogies and damper brackets are notable examples of this. What's everybody else's thoughts on this matter though?