I have used a lot of different ones and I always come back to Fertility Friend.
Pros:
easy to use
has both app and online/computer options,
Free version has everything you need
automatically will calculate and provide you with O date/cross-hairs/coverline/ based on the temps as you put them in (most other apps I've seen will record the data but you have to set your O date)
has option to override the automatic O date if you want
has option to calculate O for 3 or 4 different ways (advanced setting uses your input for CM, cervix, temps and OPKS; OPK version only uses OPKs, override lets you put in yours, plus one other setting I can't remember)
Paid option lets you do a lot more, but isn't required. They often run specials or deals where you can get a free month of the VIP version and then decide if it's worth keeping. Then they will run specials where a year of VIP is like $29 (thats the time I signed up for a year is with the big price dip) but VIP is NOT required to have the important stuff.
Easiest chart/graph I've used. Other apps have "prettier" charts, but this one is easier to read and I consider it more accurate (as a math teacher I'm really picky about the way that graphs are set up).
CONS:
not as "pretty" as some other apps
Some of the fun features like chart compare (where you can compare your charts with other womens) require the VIP function.
I've also used: Ovia, Kindara, Glow (awful - don't EVER USE IT), and one other (can't remember name) and FF is the one I always go back to.
Thanks ladies 
