Speedsleev Ranger. It comes in 3 sizes, 5 different colors, does not attach to the seatpost, and stays in place(no sway). It also uses simple velcro so it is fast to mount or remove and wont fail(like Silca Boa styles used to). The velcro is nowhere close to contacting lycra shorts too, which is important.
I have had this style bag on all my road bikes and gravel bike for almost 3 years now. The large size fits a big 35mm tube, multitool, tire levers, patch kit, chainlink, and co2 inflation.
They are often on sale for under $25.
