Wasn't sure where this should go because I don't want it to be strictly Gundam designs. But yeah, is there one single mecha design you'd consider your favorite?
For me, I'd say the C.L.O.D. from the cartoon Xyber 9. I suppose it has a very war machine sort of aesthetic, but I think it has a lot of character to it in being a sort of mechanical gorilla. The colors are muted but not boring. It feels rather crude in a good way, especially with those large claws. It has to walk on them like a gorilla and even needs treads on the feet to help it along. There's a strong sense of weight and power to it. And I think all the weapons being mounted/internal adds to that: the C.L.O.D. is the full package, it's own arsenal. And I think the general look and feeling of the C.L.O.D. helps it really fit the post apocalyptic setting well too.