what is the top speed of a sail fish?

The short answer is 'very fast'! Measuring animal speeds is very difficult as of course being able to obvserve them properly is hard enough, butto do that at high speeds becomes much harder. In water of course this is hardest of all, especially with an animla as fast and as rare as sailfish. However, there are reasonable accurate records that put the speed at around 60 miles an hour during a high speed burst! (thats just under 96 kph). This would actually make them the fastest powered animal around, faster than the cheetah and only bettered by falcons that cheat by using gravity to make their dives fater. When you consider how hard it is to move in water, this is an incredible achievement.

Just to add a touch, I think that the whitethroated needletail (Hirundapus caudacutus) is clocked at somewhere around 90mph in level flight, making it the fastest powered animal and many common ducks hit the 60mph mark, although, saying that, 1) I can't find any reliably sourced answers on a quick Google search and 2) as Dave says, accurate measurement of speed in wild animals is extremely difficult...

Sailfish are fast indeed ... but not quite the fastest powered animals around.  According to the table at the front of The Guinness Book of Animals Facts and Feats (which is pretty solidly researched), the fastest speeds not involving stoops, dives or wind  assistance are "100+" mph for the Alpine swift Apus melba, 99 mph for the Magnificent frigatebird Fregata magnificens and 88 mph for the Spurwing goose Plectropterus gambensis.

The next few entries represent flying birds as well: the fastest listed non-flying animal is indeed the sailfish Istiophorus platypterus, whose speed is given as 68 mph, with the cheetax Acinonyx jubatus as the fastest listed land animal (and second fastest non-flying animal) at 62-65 mph.