I use InstantProxies and it works perfectly on Neopets. This said, I am in the US, and if you're outside the US, your mileage may vary.
If you don't always connect from a static IP address, InstantProxies won't work for you, since they function based off IP authorization.
The only other one I had tried apart from InstantProxies was LimeProxies, they were fine, I just liked the support I received at InstantProxies better.
This review site seems to aggregate the proxy competition fairly well, you may find a few other options here:
(you need an account to see links)