i put expensive vwr springs (lower 20-25mm) (£220) on my mk6 gti and they aren't bad (ride quality is a bit stiff) , i'd personally go for VW driver gear springs which can be ordered from the dealer infact (a quick google will show mainly american posts) but these are genuine eibach springs made for VW, meant for 2.0 tdi mk6's and will lower well, last time I checked £115 at dealers for eibach springs? bargain! these lower around 15mm all round on your gtd but is noticeably lower, these also have proper VW part codes so no insurance hassle or warranty hassle.
these 2 springs will be the comfiest I think from my extensive research into not breaking my back
You could look into eibach prokit which is even lower than the 2 springs above, but will impact ride quality more. H&R's ive no experience with but some swear by