Since the 1980′s I have used many small camp stoves, but none compares to the Svea 123. This stove is famous for working well at high altitudes. It is light, reliable, simple, and will boil water fast. It does lack a simmer setting, even on its lowest setting. And it can be a bit much for some things, but when camping or backpacking a long simmer is rarely desired.
This uses standard white gas (Coleman Fuel) and can be refilled from partially full, giving it a great advantage over disposable cartridge pressurized gas models. It is fully self-contained which is a nice advantage over other stoves such as the Whisperlite, but at the expense of fuel capacity.
It has been made, nearly unchanged, since 1955. This is a testament to its reliability and usefulness. If I had to choose only one outdoor stove to use ever, this would be the one.
[For more information Optimus has a PDF fact sheet for the Svea stove. And, lastly, there is an excellent thread I highly recommend folks read explaining why the Svea stove is different (read: really reliable) over at Amazon.--OH]