I'm sorry, I disagree... Ailsa was a great character from her debut right up to her departure (This is only a guesstimate, as I have only seen clips of her last few years on the show via YouTube rather than full episodes, thanks 7TWO).
Leah on the other hand, her character seems to be well past her peak, she's barely involved in any interesting storylines, all she's been doing this year is attacking people with her two-bob's worth (Ryder was one of her victims). Also, she needs to move out of the Summer Bay House, she's been there for three darn years and is nothing more than a waste of space. I love Ada, it's just that Leah needs a hobby.