Can you truly be happy when you're an evil soulless vampire? I can't answer, so I didn't pick season 2.
Season 4, he's got the chip, which sucks for him. Bad. So not season 4 :)
Not season 5, because all anyone seems to do is put him down, including the woman he falls in love with.
Season 6 I don't have an explanation for.... :P
Angel season 5 is higher than others, definitely. Probably second or third.
I picked season 7 because of LATER season 7. In the first half he's mad as a hatter being tortured by The First, and he's got his soul now, VERY not fun for him. But I think in the second half of season 7, especially the last few episodes, was the time in the series where Spike was most content.

I don't think he can ever truly be happy living with the guilt of what hes done. In season two he was just happy being evil and thought that the woman he loved, loved him :)

he finally got buffy