Assuming there are separate receptacles for each light, a menorah can be any shape. However, the the Biur Halacha in siman 671 siff 4 quotes a Maharshal that lighting in a circular utensil is not a hiddur for ner Chanuka.
Kitzur Shulchan Aruch in siman 139 siff 9 says to make the lights 'straight in a row, not one high and one low'. This has become standard teaching in most schools I know. It is however taken greatly out of context. See there and the Ramma in the aforementioned siff with the Mishna Berurah #15. The only time one must be careful to light in a straight row is when you light all the lights on a single plate and there are no separate receptacles or partitions.