Entei always gave me that feel that I was against something truly powerful, something that I had to admire and take caution of.
Raikou always seemed like the one (to me at least) that I was lucky to see in my life time, and was always the one I used my Master Ball on.
Suicune though... He gave off that feeling of elegance, and it felt like there was always a connection with Suicune. I guess that connection stuck well with me, cause he often stayed on my team when I was younger, not only cause it was legendary, but because I generally loved Suicune.