Though call b/t the serves of Ivanisevic and Krajicek. Ivanisevic definitely got a lot of press and had that fear factor. He deserved both. To me, his serve was like a McEnroe with a tiny tad less spin, but maxing out at around 135mph instead of Mac's 105mph. Now that's scary.
But Krajicek also had amazing direction and power. Anyone who saw him against Kafelnikov at the US Open will attest to that. He hit something like 50 aces and there was some crazy stat about how only a few of the 1st serves Krajicek got in play weren't aces. Keep in mind, Kafelnikov was a big man at 6'3" with a long reach and was not easy to ace.
To me discussing who had a better serve b/t Ivanisevic and Krajicek is kinda like trying to analyze whether the USSR or USA had better nuclear missiles in the '80's. Both countries had more than enough firepower to destroy the others. It becomes more of a discussion on over reliance on this power and what other flaws these countries (or tennis players) have.