My little girls seem to settle on a new favourite every 2 to 3 months. We've been through Neverland Pirates, Curious George, Doc McStuffins, Ben & Holly, Scooby Do and various princessy ones etc. My older girl is currently fixed on the film Frozen (and all the songs :-)) as well as the show 'Austin & Ally' and my little one(2) is still set on that old classic Peppa Pig. You get quite used to them being all excited about one programme or other and then feel a slight sadness when they move on.
Other than ensuring they don't watch too much and that whatever they're watching is age/content appropriate I wouldn't be pushing them to watch one thing or genre over another. I think it's OK to expose them to educational or nature programmes (which they usually enjoy) but again i wouldn't force it. They usually know what they like so as long as it's in moderation and not the sole or primary input in their daily lives there shouldn't be a problem. They can never be completely shielded from societal influences whether they watch TV or not so as long as things are balanced and you're there for them on a daily basis (if possible) that's all you can do. I came across a quote once which really made sense. "If you don't genuinely listen to their small problems when they're little they'll never come to you with their big problems when they're older because to them, they were all always big problems." I may be paraphrasing somewhat but it made sense to me :-)