shopping in HK

I strongly recommend harbour city and canton road ig you would like to buy brand items. You can find the Gucci, Chanel and Louis Vuiton in this area.
I am not sure your expectation in standard in quality, but i suggest if you donot have enough time to spend in Hong Kong, try to go to "Tung Choi Street" (the Exit E of Mong Kong station) to find some cheap clothing. However, the quality is not good and private items can be found everywhere.
For better quality you can go to Exit C, the Longnam shoppihng mall. You can find most of the Local brand and Famous brand (e.g. Nike, ans, OZOC, Levis, Bread n butter, even Muji of Japan). H&M is also included.
As Tsim Sha Tsui (Harbour City) and Mong Kong are not too far, you can spend a day for these two areas.
I would like to place some comments on other shopping malls you have mentioned too, please take a look:
elements -- Brand items and high fashion, the largest shop maybe H&M.
apm -- Chain shops, you can find same shops in MongKonk. For shopping purpose it is not worth to visit.
new century -- same as above
sogo -- Some of the brands are smae as harbour city, for example, mary quant. However, the supermarket sells special Japanese food and snacks. I suggest you can visit it if you have time. the one located in Causeway Bay is better than the one in Tsim Sha Tsui. Items suit every age group.
landmark -- high fashion with better services compare with Canton Road, As there are too much customers in the Kowloon side. (I work near Canton Road and there are lines of people in front of LV always)
festival walk -- you may be interested in LOG-ON shop as it sells funny Japanese stuffs. Most of the brands (clothes) suit customers over 25 years old and middle class.