Harilal, if by best u mean the slowest of the 3, than yes 5670 wins.
The 4850 and the 250GTS are both faster than the 5670, and by a descent margin.
THe 5670 is not without merrit, its a very short card, and doesn't require and extra power connector, so it sucks in a good clip less power than the other two (less strain on the psu). It can also do DX11, but at the performance level, it might as well not.
The 250GTS is marginally quicker than the 4850, and sucks marginally more power, in retrun the 4850 supports DX10.1 vs the DX10 for the 250GTS.
Whichever is a cheaper, thast the one I would get out of the two. If you have a weak PSU, u might consider the 5670, otherwise skip it.
Go for ATI 5670 , its Best from those 3 , with new Technologies !
Better options : 5770 < 6850 < 5850 < 6870
if u get 4 that i listed , then u will Experience Smooth & great gaming .
lol. why does 5670 is the best choice between the three? if OP looking something that doesn't need 6 pin power connector then it might be. just because the card incorporate DX11 and several others AMD new feature will not make it as a best choice
if all the cards he listed above priced close to each other then HD4850 or GTS250 will be the better choice in term of performance. 5670 is near to 9800GT/HD4830 performance but using less power than the two. if DX11 is important to you then you're better of getting at least 5770