Depends how you're going to use it... and what you'll spend.

I use a Dunlop 535Q and a Morley Bad Horsie. The Morley is good, as it's switchless, so on bypass as soon as you take your foot off it, rather than having to rock it forward to switch it off like the dunlops. However, the sound is very modern, so it works for hi-gain, but I find it a bit too much for blues.

The dunlop is excellent, and has a vintage voice. The 535 is adjustable, so you can tweak it to your ear, and style of play. I used to have a crybaby wah, which is non-adjustable but again has a nice voice to it. The crybaby is the cheapest by some distance.

Best bet as ever is to go into a shop and check them all out, and then look on the 'net, or ebay to get the best price. I managed to get my Bad Hosrie off ebay for what works out to be about $90....