Check out Kiyote’s Guide to Creating a Training Plan.
I’m working on building a modified Ben System. It’s very time consuming to create.
I chose that system after watching this video:
[edit: the video is no longer online]
I was totally blown away by the video.
Wang Feng is the current world memory Champion and he apparently uses a 2-digit non-PAO system, which suggests that the complexity of the system doesn’t necessarily make it faster. Though Ben Pridmore did break the world record again for the most decks of cards memorized in one hour.
At the moment, I think the best system to go with is the one that inspires you the most. I’m inspired by the Ben System even though it is taking forever to finish. 
I think that you can’t go wrong with a 2-digit PAO system based on the Major System.
Other people here might have suggestions about different systems.