What kind of filters should I use?

Answer for 1" filters
Man, that is a loaded question. The number one thing to remember is that the filter is for the air conditioner, not you. We have to focus on what will protect the A/C first and if we have some latitude, we can improve the filter for us. There are so many different variables that play into what filter you should use. The biggest is the size of the filter grill vs the size of the air conditioner. Your guidelines are as follows:
The filter should be 1 sqft to 1.7 sqft per ton of air conditioning. The more, the better. So a 2 ton ac unit should have between 288 Square inches to 490 square inches. The closer to the bottom of that like a 15x20 filter(300 square inches) you need to stay with a cheaper and less restrictive filter. If you have a 20x25 filter(500 square inches) you have more options. You can upgrade to better filtration without harming the unit. Some of you have 2 or 3 filters on 1 unit. Just add up the square inches of all filters on that unit to see where you fall. The filter range is as follows.
2 ton - 288 to 490 square inches
2.5 ton - 360 to 612
3 ton - 432 to 735
3.5 ton - 504 to 857
4 ton - 576 to 979
5 ton - 720 to 1224
To put this in perspective, try breathing through a drinking straw for 5 minutes. It can be done but it would suck. Now try breathing through a coffee straw for 5 minutes......
That is a little lopsided but you get the picture. It matters.

90 DAY FILTERS
I hate these filters. Let me go back to the 2 ton unit to explain why. Air conditioners are designed to move 400 cubic foot of air per minute(CFM) per ton of air conditioner. So a 2 ton a/c moves 800 CFM. It doesn't matter what filter you have, that is a fact. If your filter is on the smaller side of the spectrum, the air flow will be substantially faster likely causing wind resistance noise and whistling noises. That means the smaller filter will have to filter alot more air than a filter closer to the 490 square inch size. The more air the filter has to clean, the faster it will clog up.
Now let's get back to the 90 DAY FILTERS. If you read the fine print on the packaging, it WILL say "UP TO 90 DAYS".
Another BIG variable is our location. If we live in Montana the unit will run about 1/3 of the time as to would here in Phoenix. So we will get 90 days worth of Montana filtration in 30 days in Phoenix. Those filters are very misleading and it frustrates me. No matter what kind of 1" filter you use, please change it every 30 days in Phoenix. Now, I will answer the question. I tend to recommend a cheep pleated paper filter. Something between $2 and $5 each.