Definitely this, it's one of my biggest issues with the game. Give us a separate screen from where to select which troops to bring. The "top of the party list" thing does not work, for at least two reasons: 1) many times I've had peasants from the bottom of the list join me in the attacks, and 2) the list is not a good tool for managing your party anyway. Each time you upgrade soldiers to ones you don't already have, they should at least appear right next to the ones they were upgraded from (not the bottom of the list), and there should be a "move to the top" option.
Another, related thing I hope to see fixed, is that we need to be able to see how many infantry, archers etc are in our party, right from the party screen. Currently, to my knowledge, you can only see this from the worldmap on mouse-over, which is very inconvenient. Oh, and changing which troops are assinged to which "formation" is not saved when you save the game, they reset to default when you load.