Lomo is good for sandwiches and in sauces (tucu..good ole cerveza negra and lomo makes a nice stew), you want something with a bit of fat marbled through it. Bife de chorizo is the one. If you like more fat with more flavour you can go for an ojo de bife. Can be hit and miss.
For me, the nicest, is bife de ancho, or rib eye, you want it with the bone in. Hard to beat.