An investigator is only as good as their wits allow them to be. Paying close attention to your environment is key to picking up on those little clues that may indicate the presence of a spirit. Augmenting our five (or six, if you have that gift) senses is crucial, so I would modify my previous statement to say that good gear and one's wits a great paranormal investigator make. And on that front, here's a list of the average loadout that my team and I run with.

Flashlights- Both white light and infra-red, so as not to foul up one's night vision.
EMF detector- To help discern the difference between a potential spirit and bad wiring. Also crucial for taking baseline readings, so you can tell if readings are spiking or are within range.
Audio recorder- I prefer digital over analog. For conducting EVP sessions (electronic voice phenomena).
Low-light cameras- For filming things in the dark. Having both portable and stationary versions is helpful, though stationary ones tend to require power, and are therefore useless in UrbEx ghost hunts.
Infra-red cameras- Sense heat signatures. Requires a lot of practice to use efficiently. Lots of false positives can come off of these. I use one that attaches to my phone instead of the stand-alone version. Made by FLIR, it's about 1/4 the price.
Full-Spectrum camera- A camera that sees in all available light spectra. Is supposedly the best way to see an apparition, if there is one to be seen.
RemPods- Like an EMF detector but with lights and sounds to alert one of the possible presence of a spirit approaching.
Spirit Box- A device that cycles through local radio stations, either AM, FM or both to allow possible spirits to let themselves be heard.