I'm totally with ad_06! I love the clearblue digital tests. They are more expensive, but 7 come in a box & they eliminate the guesswork. I've been using them for 6+ months; and I still get excited every time I see that smiley face! :)
I am the type that has such a hard time with the ones with the lines, because I can never tell when it is actually +. So you might be able to tell with those. But if not there is one type that I fell in love with and that is ClearBlue Easy Digital ovulation test. It will give you a smiley face when you are ovulating, and when you are not it's just an empty circle, you only get 7 sticks, and they are pretty pricey, I'm thinking around $40.00. I hope my experience with them will help you. Good Luck.
I know as far as just the pee on a sticks brands go, that Clearblue easy is the most sensitive. They are about $25 or so for 7 sticks, but I think I saw them for A LOT cheaper at walmart, if I'm not mistaken.
I'm sure some of the other ladies can give you some info on the fancier kits, but I just wanted to chime in that if you just go the stick route, from everything I've read and everything my doctor told me (and what from what I've experienced!) the clearblue easy sticks are the most sensitive. I'm not sure if the same can be said for their kit though.
I was happy with the $tree OPK's until they stopped selling them, at least in my area. So I started using the Answer brand test strips with 20 in the pack for about $17 at Walmart. They work just fine but are kinda hard to find sometimes. If you can't find them, Answer has a 7 pack of pee sticks for about $11 that you can try.
Good luck!