Soa Palelei has 20 fights and 20 stoppages. He is the most dangerous finisher based on statistics. He has fought mostly outside the UFC, but it's still a crazy finishing rate. Ronda Rousey is also impressive in that she has a perfect record, all stoppages, and 8 in the first round.
Joe Lauzon and Joe Condit are very impressive in that they've fought a lot in the UFC against high caliber opponents and have a finishing rate of 96% and 93% respectively. Lauzon had a perfect finishing rate up until his last bout versus Mac Danzig.