Flattering will depend on your definition of the word, and your body shape and size.
Do you want it loose or fitted, cropped or long length? Where are you planning on wearing it, just to the gym, just in your home for slobbing, or somewhere you don't mind being seen and want to look fairly decent?
Hoodie as in a pullover with a hood, or a zip fronted hooded sweat jacket?
Do you mind a logo?
I personally have the Acne hooded sweat top which is £££ but superb quality and fleece lined. I wouldn't wear it for the gym though, I wear with jeans and hi-tops.
www.acnestudios.com/uk/en/ferris-face/2HK173-900.html?categid=woman-sweatshirts (ignore the rest of the models outfit, Acne have bonkers styling)
For the gym and with leggings I have the mens version of this, heavy loopback cotton, but you gotta like a logo!
www.adidas.co.uk/trefoil-hoodie/CE2408.html
I prefer Adidas to Nike for clothes, Nike sweat tops don't wash very well IMO.
Gap stuff usually washes well for a good price (sign up for a discount code)
Avoid Sweaty Betty sweatshirts, every one I've had has faded and washed like crap.