I REALLY like Undertale, that's my favorite, but I'll list some others since that's kind of a basic answer:
Starbound - I don't see many people talk this ine, but it's a sandbox/semi RPG game with a lot of unique lore and storytelling where you explore the universe. If you like Terraria I recommend this one!!!!!
Paper Mario - specifically Paper Mario 64. I haven't played thousand year door or sticker star or any of that except Super Paper Mario. I like 64 better but Super has a lot of unique features!! also I LOVE RPGs....!!!!
Little Big Planet - I played and adored every LBP game but I played LBP2 the most. It is the biggest most memorable game of my childhood and I will probably never find something that captures its magic again