Moleman.
http://i.imgur.com/F1wjjgF.png
I chuckle every time I see him, this is VERY beneficial to me.
If you haven't purchased Mystery Boxes until you open ones with Squeeky Voiced Teen and the Blue Haired lawyer I'd suggest that. Buying them with mystery boxes turns out to be cheaper than just buying them outright, it will also help your multiplier which will eventually make characters cheaper for you (through farming donuts).
Roscoe with the Ajax Steel Mill is a limited time premium character that you should watch out for then buy (hold off on doing any of his quests until you find out why, or it's a waste) ;)
Also if you play or can load your game on a iOS(apple) they often have deals on iTunes gift cards, so you can get a little discount on buying donuts with real money that way.
In the end it's up to you really which premium characters you want.
