If it’s a mk5 GTI you want 55mm.
The main thing you should decide now is whether you want coilovers or a spring and damper kit. In my experience modern coilovers (if you avoid the budget crap) aren’t the ‘track-only break your back firm’ they used to be. I’ve found numerous times playing around with mk4s and now my mk5 that often a decent set of coilovers can feel comfier than a set of aftermarket springs and dampers.
I’ve had the Koni STRT spring and damper set before and didn’t rate them. Uneven drop and very uncomfortable. Although that was the mk4, so maybe things are better on the mk5.
ST coilovers are basically what Weitec coilovers used to be. Name change at the most. They’re essentially KW dampers but not stainless. So you’ll get the characteristics of KW coilovers (rated highly by many in this game) but theoretically without the pricetag of KW coilovers. I say theoretically, because last time I checked ST themselves were overpriced - at least compared to Weitec back in the mk4 days. Which brings us on to.....
Vogtland. Same rebranded KW coilover kit. Same damper characteristics. Same non stainless strut bodies as ST. I think they use Vogtland’s own springs (they established themselves as a coil spring manufacturer years ago). BUT... priced a mile better than ST. I run them, and after I think two years now, I’m still as pleased with them as the day they were fitted.
If you’re not bothered about going much lower than stock or having the capability to adjust the ride height of each corner individually, a shock and spring kit could be a worthy option. I’d personally avoid the lower end stuff like Koni STRT. VWR aka Racingline do a kit that’s been run by a couple of folks on here who I’d trust my own car with - and they like the kit.
Bilstein are to dampers what Walkers are to crisps. They’re good. Just do your research on the different dampers Bilstein offer, because they make so many different types that will ultimately all ride slightly differently, and ride quality is a subjective thing.
I’ve no experience of H&R Comfort, but again, H&R are a solid name.
If your budget is in the ballpark of OEM shock and spring renewal, do just that. Standard suspension isn’t terrible. You could pay the same money for an aftermarket kit, and end up with something terrible.
