My most loved mascara has got to be Dior Backstage, My uncle bought me some from America a few years ago and I loved it instantly. It lasts me ages and he has bought me some every year since. I'm due some more now as its starting to dry up but it has lasted me nearly a year! I know we are advised to use new mascaras every so often due to bacteria etc.... but I've never had any problems using mine for so long. It's pricey at £22 but it lasts me that long it makes it worth its money. If I were to use cheaper mascara I'd have probably spent that much (or more) throughout the year anyway.
It never fails to make my lashes longer and thicker with just two coats. I've never tried a mascara as good as Dior Backstage before and really recommend it.
Apologies for the picture from the internet, My camera on my phone is really rubbish and this awful winter darkness makes it nearly impossible to take good enough photos.