For me it is 0.3.21

Pros:
+ No forced transaction fees for tx over 0.01
+ Nice looking GUI
+ CPU and room heating feature a.k.a CPU mining is still there

Cons:
- rare DoS vulnerability
- rare bugs causing application to hang
- longer initial blockchain download

Using 0.4.1 right now but it sucks, it forces to add tx fee when sending new coins. Tx fee are bad when you are playing satoshi dice.