So, what is everyones opinion on the best rust preventative? Specifically, for firearms in medium-term storage. (a month or 2) Preferably, I'd like something in a squeeze bottle, so I can apply a little and wipe it on. Currently, I'm using Rem Oil, but it seems to dissipate pretty rapidly.
Almost as important; what shouldn't I use?
Where to start. There are so many good rust inhibitors and lube agents that there is no clear "best". The best rust inhibitor for me and my firearms is frequent use and frequent cleanings. I use Gun Butter and regular gun oil solvents and have never had an issue with rust. I conceal carry and work on some of the most humid conditions possible and have never had to really worry about it too much.
I doubt anyone here has done a study leaving his guns in the snow for a week, with different rust protectants applied. There may be no one answer to your question but a bunch of credible opinions. The newsletter Gun Tests ran such test a few years ago. Their runaway winner (out of 6 or 7) was the Smith & Wesson Lubricant and Protectant. It used to be sold in bullet-shaped spray cans. They discontinued it. Its origins could be traced to a chemical company whose name I don't remember. I procured a number of sample size bottles, and use it for hiogh risk items. So far so good. It may pop up every now and then on eBay or other resale venues. For regular safe storage, I use MPro gun oil.
If you have a bad rust problem, consider a dehumidifier and a hygrometer. Takes me tons of silica.