i guess i didnt list fargo because i dont really see it as characterizing their work. i find that film overrated and feel that it characterizes a certain segment of their work, but not their best. their darker pictures are a bit better to me - like millers crossing. all of their films are just so unique that its hard to pick one film that is all that much like the rest.
citizen kane is welles best obviously, but i recommend touch of evil as well. billy wilders "the apartment" is also good if you liked sunset blvd. not the same film by any means, but a good sample of wilders work.