Ortlieb
Saddle Bag Micro Two

What can I say? This thing is great. Yes, it is a bag and it holds stuff. It's also waterproof, super easy to open, and also to take off the bike. I own three for my bikes. The one downer is that the material breaks down where it is folded after fifteen years or so, but that's a great lifespan for a bag I think.

It holds a lot of stuff without having to cram it in. Can carry tire levers, tubes, multi-tool, cash, ID and more and when needed doesnt have to be pried out. Clicks on and off when you need to put bike on a stand. It is waterproof. The attachment to the seat rails seems very sturdy unlike a previous model by Fizik which eventually broke. So far, a great seat bag.

Great looking bag, good quality but the saddle rails on Brooks saddles are too wide for mounting bracket to fit.

It holds what I need, and is secure and out of the way.

large enough for complete kit including tube but small enough to be tucked under saddle. No hook and loop closures is great as shorts do not get that scuffed look when brushing against the seat bag. Use internal bags to group items as you may find items clanking if left loose. For century rides I tend to carry several items which are unnecessary for shorter rides but leave bag packed for either type of ride.