The best allergy relief air purifier (at least based on Consumer Reports) is not necessarily the best one for you. That's simply because room size, and the specific types of allergies you have, can change the brand that fits' your needs best. Purchasing the most highly rated model, or even the most expensive allergy relief air purifier on the market ($1200), will not guarantee success in allergy relief.
The way I removed allergies and asthma from my body was a combination of air purifiers, herbs (for cleansing the liver), and other nutritionals to build up my immune system.
However people allergic to pollen are also likely to be allergic to Mold and Mildew - something HEPA will not help with. For Mold and Mildew you'll want to add UV (ultraviolet) light. If you also have chemical or fume sensitivities then you'll also want Carbon or Zeolite to capture gases.
When I had allergies the greatest relief came from an allergy relief air purifier that had multiple technologies, giving my immune system as much of a break from as many problems as possible. EX: the most expensive air purifier on the market, the IQAIR Purifier (starts at $899), includes HEPA but does not include UV, and adding Carbon will cost you even more. The HEPA is high quality, but unless you have a major lung condition you would be better off with an air purifier that includes more technologies. For that matter you could buy one for the living room and bedroom and still spend less than the IQAIR.
1 - Most HEPA replacement filters are overpriced. This is how many companies make their money and how you may end up paying too much in the long run. The Comparison Chart gives you an idea of what manufacturers charge.
2 - If you plan on using it in the bedroom (and for allergies I suggest you do) make sure the low fan speed runs under 24 Decibels. Otherwise you'll have a hard time sleeping with it on.
3 - if you buy an allergy relief air purifier that includes a washable pre-filter (which I suggest because it catches hair and dander, plus it will extend the life of the HEPA filter) wash it every week. The level of dust and hair will give you an idea how much stuff is in the air.
4 - I run my bedroom and office allergy relief air purifier 24/7. It's low on energy, gives me added energy because of the negative ions it produces, and makes sure I am continuously cleaning the air I breathe in.