So far I've played 3 solo games: Knight, Dragon, and Goblin.
What I find most interesting is each solo game is fun thanks to the solo variants written for each one. I also like that there are various difficulties that you can use if you find the game too easy or too difficult.
My favorite solo right now is probably Knight because I love RPGs. This really feels like I'm dungeon diving and leveling up a character. I'm collecting loot and smashing crystals all while trying to survive the goblin ambushes. Then, if I'm unlucky, the cave will start falling in on my Indiana Jones style and I'm bolting toward the exit with my bobbles hoping to survive!
I will admit I like the Knight the best so much so we might have an announcement about it soon.