Traviata is a must if one hasn't seen this production live before. It is beautiful. The DVD doesn't capture the audio, visual, or emotional experience. Hearing the Met strings play the prelude live is divine. One of the best productions the Met has and this is the last season they're doing it. All the other productions will likely be back. The music, the cast, the production - it's all great.
Onegin is nice. Below average production. Music/plot is good. It's fun to see Netrebko perform. If you've never seen her perform live, that might be a good choice. The old Russian ladies are going to be attending in force because it's a Russian opera with Netrebko. There is a high probability there will be people talking nearby in Russian during the performance. You will need to shush them. Not joking.
Romeo et Juliette. Damrau is the only reason I'm going to go. The production was in Chicago and it wasn't praised. Not a big fan of this opera TBH. Despite having Damrau - who is going to be great, I don't think this is the one to pick. As somebody mentioned, Puritani has Damrau and Camarena - both are electric. Production is very old-fashioned but that's not going to matter with those two singing. Damrau is supposed to be in Tales of Hoffmann at the Met in the future singing all four roles. That would be a great show to see her in.
Der Rosenkavalier - If you're not sure about this one, maybe don't choose it. It's a beautiful opera but I've heard a few people bitch that it's sitting around bored for three hours waiting for the amazing trio at the end. It's psychologically very powerful and has the potential to fuck you up but, like many Strauss operas, you've really got to be into it the entire time and follow what's going on. If you go, maybe go to the last one of the run because it will be Renee Fleming's final performance of the role. People cry at the end of Rosenkavalier anyways so even more tears that night. The entire run of Rosenkavalier will be an event and will sell out. Always nice to be at the Met when it has that sell-out buzz.
Guillame Tell has an amazing cast and it's not something that is performed that often. If you're a hardcore opera person, than could definitely be a good pick. If you're not and you're not a Rossini fan, it could be a long night. It is gorgeous but is not the musical and operatic perfection that is Barber.