For me, the reasons are:
She has never at any time allowed herself to stoopp to the level of those who were bitching behind her back and generally treating her in a horrible way without any justification. Most people would've cracked, bit back or ended up in a teary mess. But Estelle took it all in her stride, kept her humour, and never once fell to their level.
I actually genuinely believe she is entertaining, good-humoured and far from boring! I am actually speaking as someone who liked her from early in the series by the way!
She has shown consideration and care for the other housemates, even those she knew were bitching about her and back stabbing her. Just a couple of examples that come to mind, who cooked the food for Layla and Sam when they were in the trashed cq? Who was the one who ensured Ben got his hour of music even though he'd been horrible to her that week?
Estelle is by a long way the most deserving imho!