I'd have to say Children's Pythons have been the most rewarding, at least out of what would be suitable for a beginner. I've also absolutely adored Water Pythons and Death Adders, but Water Pythons get large, aren't necessarily the best choice for everyone, and are very consistent in appearance compared to other options. Death Adders are spectacular snakes, I have enjoyed them immensely, but clearly a deadly snake isn't for most people.
Children's Pythons are just brilliant (I'd include maculosa and stimsoni in the group). They're very easy to keep, a very convenient size, a real pleasure to work with and these days come in a very impressive variety of colours and patterns. There are multiple good options in terms of types and morphs and this largely comes down to personal preference, but for me, the best morph is marble. It's very beautiful, comes in a wide spectrum with some having background colours of white, cream, yellow or orange (I'm absolutely thrilled with the yellow/orange ones we've been developing!) and the darker pattern can be black, brown or reddish, it has two different elements (a bold pattern and another layer of 'peppering' I call it, which increases as the snakes age). Both elements of the pattern vary a lot. I've included a few pictures of mine.
Chondros are mentioned above, they're undeniably beautiful and I absolutely love mine, there are plenty of good things to say about them, but they're also a very difficult species for beginners and there's no Australian python with more specific requirements. Definitely worth a look at as your third or fourth python, I love mine, but I wouldn't consider them at all appropriate for a newer keeper. I've had to save several in friends' collections which would have died from prolapses and in my own Chondros I've had sloughing issues which as a newer keeper I would have been unable to deal with, and in such a sensitive species with such thin delicate skin these can even be fatal. No problem for me after 25 years of working with snakes but not something I'd want in the hands of a new keeper. If you want to get Chondros by all means get some, they are unique and spectacular, but I strongly recommend keeping some other pythons for at least a couple of years first - nothing more heartbreaking than the death of a pet, especially when a child is involved. Another issue with Chondros which is not an issue to me but is important to most keepers is that they are generally not appropriate for regular handling. They stress more easily than any other Australian python, some people even manage to injure them while handling, especially the tails, they are more nervous and prone to biting than most other pythons, and size for size they have a surprisingly painful bite compared to other pythons. My first few Chondro bites which were from small ones amazed me, I'd been bitten probably hundreds of thousands of times by snakes if you include hatchlings, and usually something like a Carpet or Children's Python of the same size won't really hurt, and while it didn't exactly bother me it was definitely painful and drew blood in a way I just wouldn't expect from other pythons of the same size.
Anyway, enjoy some pictures of some marble Children's Pythons
Of all the species and morphs I've worked with over the last 25 years, this would be my pick