I love Thai peppers for their "fresh" flavor. The very hottest of these (according to the Scoville scale) overlap with the mildest of the Habenero. The Thai is spicy enough and slight enough that unlike the Serrano it never feels like you are a munching on a salad. My own particular favorite are the green Thai peppers, because though somewhat less spicy than their more ripe rich red cousins they impart a tangy aromatic flavor which fills out the heat with a richer taste. Also, unlike the Habenero the Thai is a small and compact package that you can keep on your side as you are downing the main course without worrying that its juices will spill. The Thai is perfect for those moments when you want to savor milder flavors in your dish, but don't want to neglect the need to fill your mouth with fire.