I need a "total nerd" category. Ahem.
We've been visiting other planets for almost 50 years now! But we've visited some much more than others. Mostly, that's because some planets are much easier to reach than others... Mars and Venus are closer both in distance and in delta-v (basically, the energy a spacecraft needs to get there). The outer planets are very far off; Mercury is so deep down the Sun's gravity well that it's very hard to reach.
Just for the hell of it, I did a tally.