If you know the handle of line you want on top (e.g. because you called h = plot(...), you can use uistack
uistack(h,'top')
Alternatively, you can manipulate the order of children of your current axes directly. The following puts the last-most curve on top.
chH = get(gca,'Children')
set(gca,'Children',[chH(end);chH(1:end-1)])