If you're up for using the command line, the best tool hands-down is ffmpeg. It can convert pretty much any video format out there into pretty much any other video format, it's free, open source, and in many cases faster and better than paid-for software (I use it in preference to Adobe media converter when I can). Downside is that you need to know a little about the command line shell of your OS. Actually that's not a downside–ffmpeg is a great gateway drug to the awesome power contained within the command line.
If you're not ready to take the red pill (or is it the blue one?), then Handbrake is probably the next cab off the rank. A very useful GUI video converter that has long outlived its original purpose of being the tool of choice for ripping DVDs (shiny disks with standard definition video content from a mythical time long ago). Handbbrake uses ffmpeg tech to do its magic, and it's open source and free as well.
If those two don't do it then it's actually highly likely that nothing else actually will. As a side note, a lot of the commercial video converters out there are just graphical front-ends for ffmpeg, and if you read the fine print you might find out you're essentially paying for a free product.