Hi Wansley,
I've rented various flats/houses over the last 15 years. In all that time, there has only been one agency that have not been a total shower of barstewards.(DJ Alexanders in Edinburgh)
Private landlords have usually been much better in my experience.
I've no problems naming and shaming the worst offenders:
Homes4U.
Never, ever, ever give any of your money to these tossers. I've not met anyone who's got anything good to say about them and they were atrocious during the 6 months I spent renting from them
Countrywide Residential Lettings.
Very similar to Homes4U although they stopped short of entering the property without permission or with minimum notice, unlike Homes4U.
Don't get me wrong, there are also shady private landlords out there too. I'd avoid those with loads and loads of flats/houses across whatever town city you are in.
My most positive experiences have usually been when a couple have got together and one of them has let out their old place after they've moved in together somewhere else.
HTH.
Pie