Here's the secret to finding the best nasal decongestant. Decongestants affect different people in different ways, so the best decongestant for someone else may not be the best decongestant for you. Having said that, I've suffered from pollen allergies all my life, so I've tried nearly every nasal decongestant that's available. I'll share my experiences here with the ones that have worked best - but I'll also offer some specific medical information on what makes them effective.
NasalCrom has been one of the best treatments for my nasal congestion - but it's specifically formulated to address allergy symptoms. It works by controlling the problem at its source. NasalCrom stops your nasal passages from releasing the histamines (like most antihistamines), but it does it by squirting the solution directly up your nose! The one downside is you have to re-apply it several times a day, so it's obviously more difficult to administer than, say, swallowing an antihistamine pill. But I've found that to be a very effective way of delivering the medication - and quickly! I've even heard that a similar drug has also been used to control asthma. One of its biggest advantages is that it's steroid-free, so you don't have to worry about any side effects.
Flonase is a similar nasal decongestant that's also squirted up the nose - and it's also been used for controlling asthma. I remember it being very effective, but unlike NasalCrom, Flonase does contain corticosteroids. My doctor was worried about prescribing it, because he was concerned it could raise my blood pressure, and even the Flonase web site concedes the possibility of mild side effects which "may include headache, nosebleed, or sore throat."
Claritin is one of the most common "orally administered" antihistamines, and Claritin-D also includes a decongestant. Unfortunately, Claritin-D has been known to cause insomnia, and it could also raise your blood pressure. My doctor very carefully steered me towards the regular brand of Claritin, which did not contain the decongestant. Ultimately I found this was less effective in controlling my symptoms, and I moved on to other medications.
But the most important thing to remember is that decongestants have a cumulative impact. They may start working immediately, but they'll become even more effective if you continue using them regularly. If you suffer from allergies, I've even heard people advising that you start using a the decongestant before allergy season. That way the medication gets a chance to build up in your system, so it's there when you really need it!