Unquestionably, the Amelia Peabody series by Elizabeth Peters. Those books are everything I love - murder mystery, archaeology, adventure, and romance - and I just *wish* I had the talent that Elizabeth Peters had. She lived the life of archaeology and adventure that I wish I could have led, and turned it all into an incredible writing career. Literally, my dream.
Truthfully? Probably all of them! lol. I say that because, with every book I write, every year that passes, I become a better writer. I improve in storytelling, grammar, plotting and characterisation. Every experience - both in reading and writing - makes me a better writer and author. So every time I write a new book, I can see the improvement from the one before. But, to pick just one - Evanders. It's a series about kids in a magical school. I know I can improve it, but it's finding the time to sit down and pick it apart for what is good and what needs to be completely re-written. Hopefully one day soon I can sit down to really flesh it out.
It needs to have heart. It's that simple. If the author doesn't believe in every word they write - whether it be sci-fi, romance or fantasy - then it won't work. To write and write well, you need to feel what your characters feel, be connected to the plot, the people and the events. Without that, it's just words.