Threads like these always turn into flame wars so I'll just say the following.
A. The point of the thread is discussing whether or not Nagato is undefeated. It's not about hypothetical battles such as if Nagato fought X. The topic is a discussion of whether or not Nagato was canonically defeated in the title.
B. He was not. Nagato [unlike Pain oddly enough] has never lost a single battle. I will now explain why people who believe this are justified in doing so by the canon itself.
1) While it is true that Jiraiya could have defeated Pain if he knew Pain's secret, it is highly unlikely that he would have been able to defeat Nagato as Nagato could control life and death.
2) Naruto was able to defeat Pain only because of the following: Konoha had intel on Pain's abilities, the entire village fought Pain, Naruto fought Pain, and it was only after Minato's divine intervention that Naruto was able to defeat Pain and even then he was just barely able to do it. Naruto was in no condition to fight Nagato after the battle was finished given that he would have to deal with Konan in addition to the multitude of abilities that Nagato did not make manifest during their battle. Also, do recall that Nagato's aim was never to kill Naruto, not even as Pain, but to capture him.
3) On every forum, every member of the Uchiha army pretends that Edo Itachi easily defeated Edo Nagato. This is not true. For starters Itachi had help from Killer Bee and Sennin Mode Naruto. Also, Nagato warned Itachi, Naruto, and Bee every time he was going to use a technique. He even told them how to either counter or avoid them. Also bear in mind that Kabuto was controlling Nagato while being ignorant of many of Nagato's abilities such as the Rinnegan's field of vision and the creation of other Paths. And when Nagato was defeated, Itachi himself stated that the technique he used was a gamble. It might have defeated Nagato. It might not. All he could do was try it and his plan succeeded.
4) And finally, the line Obito > Nagato should never appear on any forum anywhere. With the Rinnegan, Sharingan, and the Juubi, Obito couldn't defeat Sennin Mode Naruto or Killer Bee. Granted he also had to fight Kakashi and Gai but let's put things in perspective. Killer Bee and Sennin Mode Naruto were fresh when they fought Nagato and within only a moves they both were in dire strains as they were nearly killed.
So canonically, no, Nagato has never been defeated. Now if this were a versus thread sure. We could say that Hashirama, Madara, and Rikudou would probably beat Nagato. But this isn't a versus thread is it? While I applaud everyone for repping their favorite characters, adopting a more objective approach to these topics will at least make you appear more rational even if it's not a facet of your overall mental state.