Since 3DS has this neat little thing called Activity Log, you can check what games you've spent the most time with (Activity Log > Software Library > Charts > Sort by Total Time Played)
I'm gonna list top 10, but if you want to include less or more then that's absolutely fine, if you feel like it you can also throw in some thoughts about the game.
1. Persona Q (95h) - Sue me, but I enjoyed it a lot, cleared both paths.
2. Ace Attorney Trilogy (78h) - First one was a bit meh, but the series gets better later on.
3. Devil Survivor 2 Record Breaker (57h) - Major improvement over Overclocked.
4. Tales of the Abyss (46h)
5. Devil Survivor Overclocked (45h)
6. Rune Factory 4 (39h) - The 3DS' Stardew Valley, very addicting.
7. Dragon Quest VIII (38h)
8. Majora's Mask 3D (36h)
9. Animal Crossing New Leaf (36h)
10. Ocarina of Time 3D (28h)