Sparkly – someone who denies the truth of the Torah which includes denying any particular Mitzvah is considered an apikorus and that is much worse than someone who acknowledges the truth of Torah but doesn’t keep it. It is ten million times worse to be Conservative or Reform than it is to by OTD or to be Chiloni!!! Rav Moshe Feinstein has a teshuva in Igros Moshe where he discusses this.
Even if the Conservative person keeps more than the OTD person it is still worse, because Kefira is one of the worst (or probably the worst) aveira that there is! There are very few things for which one loses their cheilik in Olam Haba but kefira is one of them. A person can keep nothing and still have a cheilik in Olam Haba, but if he is an apikorus, no matter how many Mitzvos he does, he has no cheilik in Olam Haba.
Just to clarify – if the person is a tinok shenishba, obviously he won’t lose his cheilik in olam haba. I am not judging anyone right now – I am just judging their actions. In Rav Moshe’s teshuva, he is talking about the Issur of answering Amein to the Bracha of an apikorus, and he says that people who are Conservative or Reform fall in this category. He says that they may be tinok shenishba, but l’maaseh they are still apikorsim. It’s possible that they are wonderful people who don’t better and they won’t be punished, but that has nothing to do with this discussion. The OTD person and the Chiloni person might also be wonderful people who don’t know better and won’t be punished for that matter.