Oh, jeez, this is a hard one... As I'm still new to this hobby, I've only kept six different species, and only two species as adults. But from those, I have to say these are my favorites:
1. Chinese gets the gold. They're some of the only species that live in my area, and if it wasn't for my sister finding one outside of her work, I would've never gotten into keeping these amazing insects. I love that they're big and they eat pretty much anything. I love how active they are, always crawling around their enclosures and begging to get let out. They are also very observant and expressive. It seems like you can tell what they're thinking when they look at you. I also like how they hop around when they're nymphs. Mine will even do back-flips from one hand to another. They're also a very beautiful species. At first glance, they may look dull and boring, with just one solid color and a green wing stripe. But when you look closely, you can see all the beauty they're hiding. There are shades of burnt orange and pinkish-grey against a canvas of cream on their abdomens. Then they have blues and reds and even yellow between their forearms. They can have green, blue, or brown eyes. And I just LOVE those little vertical face stripes!
2. Ghosts are a close second. I just absolutely love how they wiggle and dance around! I catch them dancing at each other, as if they're having a dance off. And they always sway and dance when they catch some food. It's so cute! But they're also aggressive eaters, chasing down their next meal. Watching, stalking, and finally striking with amazing accuracy. Even though they're such good eaters, they are still a communal species, which is a giant plus. They come in a huge variety of colors, and they each have slightly different leafy lobes. They love to come out of their enclosure and just hang out with you. One of mine actually sat on my arm as I ate breakfast a few days ago. They like to interact with mantids in nearby enclosures, but they never seem to look at them as food. They're small, but that only means I don't have to buy huge gross crickets for them to eat. And their camouflage never ceases to amaze me.
The honorable mention goes to the Gongy. They're beautiful gentle giants. They wiggle back and forth in such a silly-looking but cute fashion. They're big, which is nice because it's easier to see the finer details. I love the curly antennae that the females have, and how they all stick their little butts in the air. They just look like royalty of the mantis world. But they don't quite make it to the top two because they're not as easy to take care of. They only like flying food, and they won't even hand-feed. <_< You have to keep them warmer than room temperature and still keep the humidity at the right level, which can be difficult. Their limbs are so long and fragile that it's hard to hold them. They have trouble walking around on human hands. This may not apply to all Gongys, but mine is also very shy. She doesn't really like being held and she's not very active. That's a big downside for me because I really like holding my mantids. But don't get me wrong, I LOVE my Gongy! She's the sweetest girl! That's why this species makes it to the honorable mention.
WHEW! Sorry for the huge post.