I think Naruto and Frodo would be really interesting. Frodo has this burden of the One Ring and Naruto has his own struggles with the Nine - Tailed Fox. They could bond over their heavy responsibilities and Naruto could use his skills to protect Frodo on his journey.
Gandalf and Kakashi would be an amazing pairing. Both are wise and experienced mentors. They could exchange knowledge about magic and ninjutsu. Gandalf could show Kakashi some of the powerful spells in Middle - earth, while Kakashi could teach Gandalf about the Sharingan and some of the unique jutsus in the Naruto world.
Another great pairing could be Sasuke and Legolas. Sasuke is known for his cool and aloof nature and his excellent combat skills, similar to Legolas. They could have some epic battles together, with Sasuke using his Sharingan and Legolas showing off his archery skills. And they might also have some interesting conversations about their different worlds and values.