I definitely have a thing for botanic gardens, and from the ones I have visited so far, The Huntington tops the list. The Huntington Library, Art Collection, and Botanical Gardens has a long name and a lot to offer. I have visited some botanic gardens/arboretums that have not been worth the ticket price(which you do not know until you buy the ticket, walk in, and then awkwardly walk back out), but the Huntington is so worth it. The hardest part is that there is so much to see at the Huntington, it ends up being a lot of work just to see it all in one day.
My favorite sections that I think should be at the top of your list are the Desert Garden(filled with unique cacti), the Japanese Garden(gorgeous and peaceful), and the Rose Garden(such a classic). Overall you could get lost on these grounds so keep a map handy. Besides the Botanical Gardens, the Huntington also has Library and Art Collections in its name, so you also get a museum as part of your ticket(as I said there is a ton to do). The indoor exhibits are a mix of manuscript collections, sculptures, and paintings.
The Huntington also has great dining options, from their fancy Tea Room and innovative Noodle House to the more casual cafeteria style 1919 Cafe. I also spent a while browsing their giant gift shop. I would honestly go back and see if I could sneak in just to shop in their store. They have a variety of items, books, jewelry, bags, kitchenware, stationery, and decorations, that I would buy for myself and definitely as fun gifts for others. The Huntington is located in San Marino, California, about twenty minutes north of downtown Los Angeles and right next door to Pasadena. Also a bonus, the parking in their spacious parking lot is free.