Yellowfang and Cinderpelt was a lovely Mentor-Apprentice relationship, honestly. I also really liked the relationship with Dawnstripe and Tallstar- it was really heartwarming. Another heartwarming one, imho, was Cinderheart and Ivypool. The contrast between those two and Lionblaze and Dovewing was a fresh thing when I got tired of listening to Dovewing's whining.
Also, Cloudtail, Brackenfur, and any of their apprentices. I love how both cats genuinely care about who they're teaching, while still being a mentor. Brackenfur did that when he was given Hollyleaf as an apprentice, and it was really nice.