I was checking animation packages for my TailLigths video
and it seemed that Toon Boom
was the industry Standard, but wasn't that affordable ($300 - 2k) , the next one I considered was Anime Studio
($50- $300 ), that looked promising, I've learned to love Manga Studio (from the same company) , but the learning curve was steep and the UI is from the 80's.
I ended up using my trusty photoshop CS6 , which has a simple animation panel with or without keyframes, it is not as nice as the above, but I already had it, It is mostly just a timeline to post keyframes ( my video uses simple tweening) , but if you are importing keyframes from say illustrator or somewhere else, it is probably the simplest and chances are you already have it.
I did a lot of Flash/Actionscript in my time, I would stay away unless you need to add some programatic/interactive animation.