For immediate relief, I use Sudafed. I can usually hear/feel everything open up within an hour. Long term, using an antihistamine daily will help. Lots of OTC choices - I use Flonase right now in addition to my regular asthma meds.
For a true decongestant, there's no beating pseudoephedrine hydrochloride (Sudafed). It's one of the ingredients used to make meth so you might have to sign a piece of paper at the pharmacy when you buy a package.
For allergies, Flonase and/or NetiPot work well. Long term Flonase use is not advised for kids if I remember correctly. Something about stunted growth... Be sure to use distilled or boiled water with the NetiPot. Unfortunately, rinsing your nasal passages with plain tap water can be fatal. Not kidding.
You can also knock out most allergies with Benadryl. I only take it at night because it makes me sleepy.
Don't use a netipot if you are already severely congested, it will trap the water and potentially give you a sinus infection. Decongest first (with something like sudafed, it's behind the counter at the pharmacy). I personally use mucinex-D. Once congestion has lessened a bit (and even drainage has occured) you can try rinsing with a netipot. The netipot is preventative care, so use it once daily, especially on days with high allergies. I also recommend an indoor air purifier if you don't have one.
I like Zyrtec (generic) for allergies, and I keep the generic Sudafed on hand for the days where I need that decongestant. Ditto on the Netipot. And every allergist I've seen has recommended Flonase. I'm sure it works, but usually by the time I've resorted to nasal sprays, I'm so jacked up on the other pills, it's hard to tell what's working! But I take a Zyrtec almost every day of the year and it saves me!