My all-time favorite open circuit rig are the old AGA Divator 324s — 300 Bar/4351 PSI, 2 cylinder, 4 Liters/bottle, about 80 Ft³. I first used them for bailout in sat because the bottles are only 110mm/4.33” in diameter, a big deal when getting through a 24” hatch and stowing in a crowded bell. The valve and regulator have the best protection I have ever seen. The hemispherical top-end of the cylinders have rubber bumpers that are great for lockouts or shallow wrecks. I am pretty short, and have no trouble reaching the valve.
The downsides are not many people can fill them to capacity and they are about 11 Lbs negative when full so would suck for tropical diving. Also the tapered valve threads are only 12mm/½” so O2 cleaning and inspection is a PIA. Trim in a 7mm wetsuit or drysuit is excellent.