Hands down, TV series is my favorite.  It's probably my
favorite anime- period.  The second OAV has some really 
good parts-
The artwork in an OAV is typically better than in a tv 
series- the style also changed a bit from the beginning of
the T.V. series to the end... the style from the last few 
episodes of TV looks almost exactly the same as oav.
OAV as a rule has pointier noses and narrower eyes- but 
some cels are impossible for me to distinguish without 
knowing the scene the cel is from.  Also, the clothing does
change between series, with pastels dominant in OAV.
All the cels on my page are from the TV series.  
-aion, who does still want a nummy Tasuki from oav2 in that
white shirt... ;)