Nanoha StS is more tricky imo, with those funnels that drain her life. So does her Starlight Breaker, though its threat level is also demoralising. I also enjoyed playing with Hayate - as long as the partner could keep aggressive melee attackers (like Kirara) at bay that is. She's a solid bombardier battleship that just fires here and there, decapitating people throughout the process, not excluding your own partner >_>
I vouch for Kirara for #1 simplicity. She's quick, fast, and while she deals rather light damage her combos are awesome. Too bad her - uh what's it it called again, the limit break thingy for MBA? - is too easy to avoid.