There are lots of amazing wahs out there but the crybaby by Dunlop seems to be the industry "standard" if you will. It's available in quite a few different options which can get confusing, but then again also adds to the personalisation of your sound.
I would say a good idea would be to go here
http://www.guitarforum.co.za/youtube-guitar-videos/the-crybaby-wah-the-pedal-that-rocks-the-world-(documentary)/ watch the documentary and get a bit of backdrop on what people searched for in their wah etc.
Then if possible the 3 I'd recommend looking out for are the 535Q which is just a nice adjustable throaty wah, the Zakk Wylde crybaby wah which is less adjustable but I believe it might be cheaper and it's also one of the most transparent ones I've ever tried, and the Dimebag Darrel wah which has a whole EQ section you can tamper with along with a Q control and volume boost and adjustable feel on the foot pedal so it can be as loose or tight as you want ?
I'd urge you to try and find any of those 3 new or second hand, but that's just my opinion. There's also the wah range from Morley that are killer but sound and operate in a different way, I believe they are also spring loaded which prevents you from being able to leave it in that half cocked position to sweeten out the mids...