You will have a tough time trying to decide what car to buy if you do not first narrow it down to a size of car. You need to determine price you can pay, space (family or just one/two person), economy required etc. etc. then you will have a better idea of what you need to be looking at.
IMHO........
Ford Ka - City car, very small, fine if you will be mostly doing town driving. Fairly good car but showing its age at this stage. Probably due for replacement in the next year or so. Cheapest of the lot here and least features.
Fiesta - supermini, bigger than Ka and an excellent car by all accounts. 1.4 diesel is meant to be v good. Along with the Yaris it is arguably the best of the superminis. same price ballpark as the corsa, similiar amount of features.
Corsa - supermini, not as good as the fiesta. Personally do not think much of corsas from past experiences. There are better superminis out there so no reason to buy a corsa. (much the same features as fiesta)
Focus - small family size, Way bigger than the Ka, if you need the space of the focus then the others are likely to be non runners. Excellent car and probably the best in its class. Both the new shape and previous generation focus are good buys. Most expensive of your choices and most features. Last generation focus was extremely reliabel so new one probably will be also.