Right Emma, i test all the time, although i never (hardly ever) get ammonia or nitrite readings, do lots of water changes and have good filters, but still like to test, its another part of the hobby i find interesting and makes me feel like a scientist and i think it impresses my son lol
Here's my take on test kits, just my experience only, but i have used quite a few in my time
AMMONIA
I like the Nutrafin NH3/NH4 kit, it comes with an Orange label on the pack, not the Yellow label that Nutrafin also do. The only draw back to this kit, it you have to wait 20 minutes for the results. But from all the ammonia kits i have used i find this gives the most detailed readings, i.e. you can really tell if one tank or your tapwater even has a slight presence of ammonia, whereas other kits will all read the same level. I have not found the API one very helpful (coluor chart wise)
My personal tip for testing for ammonia and actually this applies to Nitrite and Nitrate as well. If you have more than one tank, always test tanks at the same time and get some extra test tubes, then by standing the results next to each other you can get a clearer indication of the colours, and match them against each tank, to see what tank (if any) is performing the worst in terms of ammonia/nitrite or nitrate. If you only have the one tank, test your tap water at the same time, just so you get a contrast in from the tester colour kit chart.
NITRITE
Much of a muchness really in my experience. I like the Nutrafin one best as if you have undetectable nitrite in your tank the test tube solution will stay "clear" i.e. the colour of normal water, then any nitrite readings will gradually make the solution turn more "pink", i find this more useful, as its easier to tell the difference between "clear and pink" than various different shades of yellow or red which some nitrite kits use on their colour charts. There is a company called JBL that do an excellent Nitrite test kit as well. I have aslo used the API one before with no problems or difficulties (althogh this is reading nitrite levels in different shades of blue, but easy enough)
NITRATE
Always a bit and miss. Testing for nitrAte is always a bit less reliable. I think others also experience this. In contrast to their Ammonia and NitrIte testing kits, i find Nutrafin's NitrAte kit complete pants, utterly useless. The more and more i think about it, the more useful i have found the API nitrate kit, it has always given me results that make sense (i.e. tanks where i would expect higher levels have higher levels on the test results). It's crucial you follow the exact instructions for test kits, but especially for Nitrate. If it says shake vigourously for 30 seconds or whatever, that what must be done!!! the API has run into some bad press on the this forum recently, couple of cases, but its been fine for me.
Oh and i found the Tetra NitrAte kit good, but expensive, and it uses powder as well, which can be tricky. Not sure if this means a more accurate result though?
PH
Any kit really, i dont have a favourite
HARDNESS
I dont have much experience with this, just done it every now and then. Cant even remember with what kit.
EDIT: oh, and liquid kits all the way for me