Easiest Characters to Hardest (IMO):
Kratos - He's got great range and a multitude of attacks that generate a lot of AP. Add to that that his chains give him ungodly large hit boxes, and you've got a really easy to use character. Also he's the only chracter with a level 3 that can last through 2 whole respawns, giving him the possibility of 3 rotations in kills with just a single level 3
Radec - Doesn't need to do a lot of comboing or hitting to get a lot of AP. His range allows him the ability to get meter quickly and safely, without having to build unbelievably long combos.Also his primary attack (snipe) generates 30 AP which is almost a third of a level 1 bar. His level 3 is also awesome. If you can't go through 2 full rotations of respawn/kill, then you're doing it wrong.
Sweet Tooth - His movement and speed take a massive adjustment if you come from Smash or Marvel, but a 3 hit combo can put you just 30 AP away from a level 1. He also gets 30 AP per character per mine. Considering the fact that it should be his most used move (next to his shotgun which also generates 30 AP) it takes very little time to start getting kills.
Fat Princess - Now, as far as play style, she's got the hardest (imo) BUT her level 1 can take her a REALLY long way. I've seen a FP in a FFA match get 8 kills with 3 level ones. Point is, she doesn't have to work too hard to get kills.
Parappa - Very hard to learn how to get in with this character a lot of the time. If your playing a scrub, you'll be able to do just fine. But if your playing anyone who knows what they're doing, you won't be getting in close without a gross amount of effort. That being said, once you get in with him, he's a **bleep** meter building God. That being said, the reason I ranked him one of the more difficult is that you HAVE to learn combos to be successful with Parappa. And while they are easy to do, most people don't have the patience to learn them.
Sly Cooper - This character still blows my mind. He can't block, so your normal tactics of landing-blocking-rolling won't work as a defence with him. You'll need to be very agile and dextrous with his moveset. Most players I see using him spend half the match invisible and half of the match doing their F+T. Sly also has one of the best level 1's next to FP, but considering how hard it seems to be to build meter with him, it definitely puts him at the bottom of the list in terms of ease of use.
Not everyone will agree, and that's fine. But I've spent a considerable amount of time on the game now and have seen new comers try to learn it. This list seems to be pretty accurate.