Personally, Nautilus is my file manager of choice. It has plenty of built in features, and anything that isn’t included, I can add it myself with Nautilus Scripts. However, while not bloated by any means, it is a little heavier then a plain file manager needs to be. If you have decent hardware, it will be fine, but if you a lower-end setup, or want to squeeze out every drop of speed, you may want to consider an alternative. While their are plenty of choices out there, here are two of the most popular that I’ve had experiences with.
This is the default file manager for XFCE, a desktop environment designed to be lightweight while not being bare-bones. In many ways, XFCE is similar to GNOME, with a smaller footprint (incidentally, it also natively supports things neither GNOME nor KDE does, such as built in compositing). If you want speed, you could ditch GNOME entirely for XFCE, but if that isn’t an option for you, you can just take Thunar.