We recommend NEBExpress Competent E. coli (NEB #C2523). This is an E. coli B strain similar to T7 Express and BL21 (DE3), except that it lacks the T7 RNA Polymerase (the pMAL vectors use the E. coli RNA Polymerase). NEB Express, T7 Express (NEB #C2566), BL21 (NEB #C2530) and BL21(DE3) (NEB #C2527) Competent E. coli all give similar results – the presence of the T7 RNA Polymerase doesn't seem to have any effect. Other successful strains include NEB 10-beta (NEB #C3019) and NEB Turbo (NEB #C2984) Competent E. coli. One can start with NEB Express, or with whatever competent cells are readily available, and then try another strain if a problem with expression or purification develops.