There is no best place as it depends on your body shape, bump size and leg length.
I have some Gap leggings which I brought to do exercise in. They are advertised as mid-calf length but come under my knees. I had to start wearing them at 14 weeks as my normal exercise bottoms dug into the bottom of my bump, and while the material is good I have to still pull them up 11 weeks later.
I have the cheap George from Asda leggings. They are ankle length, while the belly and back are loose, their cut means I don't have to keep pulling them up. The material is thin so I doubt they will last more than this pregnancy.
I have JoJo Mama Bebe jeans which I started wearing from 17 weeks. The belly band was extremely loose first but now, while still loose particularly at the back, is OK. I've been warned jean belly bands rarely last more than one pregnancy.
I wear all with long loose tops, some of which are maternity, or under dresses.
I've track suit bottoms that still fit that I wear around the house if I have to wear clothes plus have been given some under bump shorts and skirts. As they are all worn under bump they feel weird as it's lower down on the front than I normally wear clothes.
Only H&M, where I have tights from, and JoJo Mama Bebe seems to have maternity items in stock. The rest of maternity wear I've brought I had to order online and return if the cut is ridiculous for me. I've read some women find their maternity clothes don't fit after 30+ weeks, so I expect over bump stuff to be loose until that point.