My favorite Glock mods are night sights (I prefer Sevigny tritium carry model scy-11), the Glock extended slide release and on my non gen 4 guns I like Talon grips.
Other than that, I pretty much leave my carry glocks in factory condition.
All the best
J
Edited to add: I also like the grip force adapter on my compact and full size glocks. The GFA allows me to have a consistent high grip without the risk of slide bite.
The best accessories you can get for the piece would be a Glockmeister Extended Slide Stop and a Glockmeister Extended Slide Lock. Those two parts are really cheap and extremely easy to install. There are tons of videos on youtube on how to install them.
If you have old eyes TruGlow Tritium sights are terrific!! The really show up in the daytime and glow at night. I have them on all of my Glocks. If you have young eyes, it makes it easy to get on target fast.