On this question, the world gets divided into two groups – the minimalists and everyone else.

If you are a minimalists you will only want the absolute essentials – two tubes and two tire irons.
So if you are a minimalist / gram counter, then buy one of those itsy bitsy tiny bags that practically fit under your seat, and you are done.

Done assumes you have a frame pump on your bike.
If not, you may need a slightly larger bag to accommodate a CO2 cartridge or two and an inflator.

Now for the other 99.9% of cyclists, there is a list of items that you will decide are either needed or desirable.
The range between needed and desirable will depend totally on your personality and your specific needs.

Let’s start with a patch kit in case things get ugly and you flat more than twice.
A boot* could be a ride saver if your tire gets cut.
A mini tool for emergency repairs could come in real handy.
Space for a cycling wallet to hold a credit card, cash, an ID, emergency medical information, etc might be a smart move.
Some folks might want room for a mini first aid kit or a medication holder.
Including a hand wipe or two for folks who like the gunk off their hands after a flat repair or emergency repair would be nice.
You will be thankful you added a packet of chamois cream if you need to keep yourself friction free on a long ride.
If you do not feel comfortable carrying your cell phone in a rear pocket, then you will want to make room for it in your bag.

If you choose not to use a frame pump, then you will want a bag that can accommodate a mini pump and/or an inflator and 1 or 2 CO2 cartridges.

On days when you are shedding clothing (arm warmers, vests, jackets, glove liners, etc) you might find it more comfortable to roll them up and put them in your bag, rather than in your rear pockets.

And you might want some extra room for power bars, drink powder packets, power gel, etc.

If you think we left an important item off the list, please drop us a line.

So you need to make a list.
Then you are ready to purchase your seat bag. 

Do we have a recommendation?

There are a really good collection of well made, well designed bags for those selecting mini to medium bags.

* A boot is placed between your tube and a hole in your tire to keep the tube from bulging out.This is an emergency fix only; change the tire when you get home.There are lots of materials that can be used as a boot.

For example:

A dollar bill
An old piece of tire
commercial boot kit

Energy bar wrapper

© (Descriptions/not image) Bike Highway; Uncle Barn’s Cue Sheet Exchange LLC, 2014 and prior year