For ease of use, AJ Horn is the best but it's not cheap. However, if you really want to get started designing BLH's and you want the simplest tool, it is the one. I have spent so many hours that it's been well worth the purchase price.
MJK is more advanced but takes more time to learn (worth it though, if you are super serious!) I use both. As mentioned, Horn Resp is perhaps the overall most popular (and free).