Not worse than Jordan Bowery, surely? They were very different types of purchases (cheap lower league punt vs more expensive 'established' second tier player), but purchase price aside, I wouldn't go with worst ever.
At the time I thought he was a decent signing, he had dominated Liverpool in a cup game the previous season. It became apparent very early on that he was useless. Somehow Pulis took him off our hands.
I don't think Gestede is the worst striker we've eer had but he is the best example of why highlights aren't much use in determining a players quality. Loads of people on here were pretty excited about him joining given his record the year before but even in the Bournemouth game it was clear that unless the ball was crossed in front of him so he just had to direct a header he was a waste of a shirt. On that basis he's possibly the most one-dimensional rplayer I've ever seen at the club and I find it amazing that he's a millionaire on the back of his playing ability.
Emile Heskey was worse. He was such a running joke it was embarrassing.
Also, in the Gestede hall of fameJordan BoweryBosko BalabanCarlton ColeLibor KozakRoss McCormackScott HoganMarlon HarewoodGrant HoltSimon Dawkins