I really, really hated normal saber (blue one). I was all excited when she went "I'll be your sword", but damn, the girl doesn't know how to follow orders. And the #lolprideandmorality is pretty annoying too. Shirou partially takes the blame, but she wasn't much better as Kiritsugu's servant. If you're going to be a sword you kill your enemies without asking, not preach to the person wielding you.
Red saber is much easier to deal with, though I still don't like it when people other than me act haughty
She's a fun character.
The best servant in this game is caster, no doubt. She's the weakest when it comes to combat, but her personality is great.
Also, for the main heroines, I'd pick Rani over Rin any time of the day~