The piece of software that I use the most is my e-mail client, with the second and third ones being my word processor and spreadsheet tools, with the presentation app coming in fourth, basically tied with my planning tool. All of my tools come from the same vendor (you can probably guess who that is :-).
Truly, while the planning software is important, it's not the tool that I spend the most time with. I need to stay in touch with all of my stakeholders, and my teams are usually at least partly global, so e-mail is critical. Status reports take up much of my time, as do change requests and document reviews for strategies and approaches and other management deliverables. Spreadsheets are the best way that I've found to analyze my financials and my progress data – that provides me with a degree of flexibility that the planning tool just doesn't have. Finally, presentations are critical to getting and maintaining executive support, as they want to know what's going on, and how you're spending "their" money.
As for project planning tools, there are no silver bullets. They all do much of the same work, some better, some worse, but your tool will not make you into a PM if you aren't, and won't ruin you as a PM if you are.