I voted for Nemesis for obvious reasons. However, the only films I can think of that I don't think would make good RiffTrax fodder are TSFS, TVH, and FC. TVH particularly because it is a comedy and I don't think a comedy would translate well to being riffed on. TSFS and FC just, to me, seem to be paced wrong and have the wrong elements to make a good RiffTrax. Some people have expressed that they don't think TMP would work well because of its slow pacing. But I think there's a treasure trove of stuff in there for Mike and pals to riff on.
Do you think that would work well? I remember back in the MST3K days the writers said they stayed away from comedies because they were hard to do comedy around. And they were dealing with bad B-movies for the most part. Here you have what many would consider a good quality comedy. Would it be difficult to riff on it, or would the humor of the movie itself get in the way?
I bet Search For Spock would make a really funny Rifftrax. Search for Spock is entertaining but kinda goofy/cheesy which makes for a good mix on Rifftrax. Insurrection and Nemesis are obvious choices too. They should steer clear of TMP (Too slow - I like the movie, but it's tone makes it riff proof) and Voyage Home (humorous movies whether the comedy works or not, rarely mixes with riffing). Not sure about Wrath of Khan and First Contact. Those are good movies, but they've riffed good flicks before and made it work.
If anyone could make it work Mike and Kevin could, there's just a lot of potential for good riffing there. Kevin's "hate" for Chekov, Sulu walking around SanFran in a CAPE, etc. MST3K did do a few comedies (Catalina Caper springs to m ind) and Rifftrax has done "funny" movies too, Fantastic Four has many humorous elements in it and they did Willy Wonka as well. So, yes, I think it could work well.
I think any of them can work well. TVH would be difficult for the reasons stated above. Granted, they've done "humorous" movies before, but TVH's driving force was the comedy. If they ever end up doing it, it'll probably be the last one out of difficultly level. I would love to see TMP done. I like the movie, but there are just some great rich material there. Plus, with the long dialog-less beauty SFX shots can allow Mike and others to go on with one of their random rants. Many may not like them, but I think they are hilarious (I was particularly fond of Mike's "how to make a wall look like outer space" rant from the GEN riff).
I don't think TMP could work. It's just too slow of a movie. And as much as I DO enjoy the rants during slow periods I'd think TMP would be nothing BUT rants becuase 50% of that movie is down time. Second to TVH, TWK would be a pretty good choice.