Right now, it's Calibre because I just got a Kobo eBook reader and it's so great to be able to install pretty much any format of book onto my device and convert it if it's a format the device can't use. And even convert it if the book works better in a different format.
Caliber is truly amazing, but Kobo support is… Odd. I love my Kobo for comics because of the color screen, but uploading .cbz files is an obtuse process. Kobo readers won’t natively read metadata from .cbz files, but you can manually push the metadata to the device’s database. But in order to do that, you need the file to actually be in the database, which doesn’t happen until after you unplug the device.
So to get a .cbz file working, you need to plug your Kobo in, upload the .cbz file(s), disconnect your Kobo, let it index the file(s), and then hope to god that it actually shows up on the device’s library when you plug it back into your computer so you can manually update the metadata.