The Arduino Uno R3 is perfect for those starting to learn how to do Arduino projects. It's easy to use and a pretty nifty, quality board.
Ranked in these QuestionsQuestion Ranking
Pro Reliable, even for long periods of time
Unlike some cheaper clones, this board is very capable of running continuously for long periods of time with no issued.
Con Advanced users will find it's insufficient
This board is very good for beginners, but once users start getting more experienced, they'll feel that what it offers is insufficient.