For block diagrams, graphviz is the fastest for me. If I need anything more complicated than node->node relations, pretty much any editor akin to visio is still

For block diagrams, graphviz is the fastest for me. If I need anything more complicated than node->node relations, pretty much any editor akin to visio is still faster than doing it by hand and looks much prettier because I have infinite space and can rearrange things on the fly. By hand is useful when all I have for collaboration is a whiteboard, though.