A Zetec or a Duratec. The 2.0 SPI single-cam 4-cylinders are unreliable.
The 2.0 Zetec is olllld tech, cast-iron, very reliable - but has a timing belt that needs to be serviced every 90,000 miles or so (although it's not an interference engine, so it's not a deal-breaker). The all-aluminum 2.0 and 2.3 Duratec use a timing chain instead, and are probably the better choice of the two.
Every Focus from 2005-on used a Duratec, so unless you are looking at a 2000-2004 model, it won't even be a factor.