I my opinion, Anki is the best:
- the friendly interface;
- the media options (recording speech, Ctrl+C images, videos support etc.);
- the algorithm Anki used is the same of Supermemo 2 (but if Anki, in future, use a neural network, with seven memory classifications, like Fullrecall, it would be better, I think), probably because that Supermemo patent has expired;
- It seems that the program are always being improved, there are plugins etc.;
- Anki is available for G1/Android Google smartphone (http://ichi2.net/anki/wiki/AndroidAnki
- Anki is entirely free (there is no "premium" version).
- There are the options of "cloud computing" (online) and desktop (that I use) computing.
Fullrecall is good, it uses a neural network, it is simpler than Supermemo (with this you spend more time learning about the program then studding your study object), but the interface is poor (like softwares of 10 years ago) there are less resources than Anki. Maybe the premium version be better, but I prefer use Anki and make known this software (and support it with a donation, of course, for stimulating its improvement).
Supermemo: a software that Piotr Wozniak has created for himself (and other computer scientists...).
(forgiven my English, I'm Brazilian, native Portuguese speaker)