mass - What type of star does theory predict should be the most massive?

Since you phrase your question "What type does theory predict?", I guess the answer must be the so-called Population III stars, which are thought to be the first generation of stars, born from zero-metallicity gas. With no metals, it is difficult for the gas to cool. The mass of a star is given by the Jeans mass of the collapsing cloud, which is proportional to $T^{3/2}/rho^{1/2}$. This temperature dependence means that the cooler the cloud can be, the smaller clumps the cloud is able to fragment into. Hence, if it's difficult to cool, the cloud has a large Jeans mass, i.e. only large cloud collapse and form stars (this explanation is rather simplified and doesn't take into account stuff like shocks, turbulence, etc., but captures the basic physics).

Moreover, without metals to act as absorbers, the radiation may escape the star more easily, i.e. without interacting with the stellar atmosphere, so mass losses may be less significant and the star is expected to maintain its mass.

Pop III stars are expected to have masses of several hundred to 1000 $M_odot$. With such large masses, they burn the fuel fast ($sim$$10^6$ yr), but their spectra are extremely hard, and they should be able to ionize helium, so one way to get a hint of the existence of these guys would be by detection of the He $lambda$1640 Å line.