http://www.jewishvirtuallibrary.org/jsource/Judaism/Karaites.html
http://www.karaite-korner.org/karaite_faq.shtml < this page answers your question completely
Karaites are oldest according to the site above.
EDIT: tickle..I really hate to speak ill about another poster..but ST is giving you INCORRECT information. He has lately been posting some really offensive insulting and false things about Jews in several posts. In one he claimed that Reform Judaism falsely leads to cases of incest and adultry ( mamzers (sic) for mamzerim, the Hebrew term he meant to use ) I would take anything he says about Judaism with a grain of salt. See this previous question and the Jewish reaction to his ill mannered lashon hara (meaning evil tongue) here: https://answersrip.com/question/index?qid=20071203063617AAy1ALi&show=7#profile-info-AA10020540
He is also incorrect about the oldest sect of Judaism. The largest sect of Jews remaining today comes from the Pharisees from the second century BCE, however the Karaite sect is older, even though it is a very small sect. If you read here..the Sadducees were considered following Karaism as well , read their information ( It is always good to learn about a religion from the source..adherents OF a particular branch or sect ) http://www.karaite-korner.org/history.shtml
Here is also a little fyi for the uninformed.
What is the legal definition of a "mamzer"?
by Rabbi Naftali Silberberg
A Mamzer (a.k.a. bastard) is the product of incestuous relations or adultery. A child born out of wedlock is not a mamzer.
Besides for the fact that a mamzer may only marry a spouse of the same ilk, a mamzer is a full-fledged Jew in all respects. In fact our sages tell us "A mamzer who is a Torah scholar takes precedence (in respect accorded, etc.) over an ignorant high-priest!"
the above was from askmoses site
I am not here to argue with one who answers in a manner to disrupt, insult and confuse. .I believe the libelous efforts directed to me that were undertaken are undone by the evidence at hand here already. We don't do this in a vacuum, others read these answers and I certainly hope that people who are interested WILL go to the jewfaq.org. site. ( There are several pages on this topic there that support what I've instructed thus far) I have often recommended it and http://www.whatjewsbelieve.org/. Both are sites I've used to help teach people about Jewish belief over the past decade.
I had already given the jewishvirtuallibrary link at the beginning of my answer before any edit.
I never claim to be an expert on anything, however, Judaism is something I am quite familiar with and if I see someone misinforming another and I'm certain that it is happening, it is my right to provide the correct information. I will do so without ad hominem. And as always..I ask..do NOT TAKE MY WORD alone for anything I say about any aspect of Judaism or Tanakh. I will provide references for people to learn further on their own. I've a wealth of hundreds of books on Judaism and Jewish history and ancient history passed down over the course of more than a hundred years and I've added a great many of my own books.
There are a great many excellent web refereneces available too, and I've got dozens of them bookmarked and happily share them if you are interested.
I like questions that challenge so I learn, too. But it saddens me more than angers me to see anyone who claims to speak for Judaism enagage in behaviors that bring shame to Jews.
Shalom