IMO the only real answer to your question is "the guard/sweep that your opponent is giving you". Open guard/passing the guard are easily the most complex and important positions in Jiu Jitsu. There's no such thing as getting good at one open guard because they all work off each other. If you're not working X, DLR, RDLR, Inverted, 50/50, Sit Up, Spider etc... your guard has major holes in it. The key is knowing how these guards intertwine. As far as I'm concerned, passing/sweeping should be 90% of your focus in training.