They're all cool in their own way, but Suicune stands out the most for me. I just love its design, and the fact that it was the star of both Crystal and the fourth movie (along with Celebi). I also liked how Eusine was always chasing after it, and his looks somewhat resembling Suicune's.
I like all three, but Raikou is my favourite. I like its high Speed and Special Attack. Suicune just can't hit hard, and I really disliked how Entei was unable to learn Flare Blitz naturally in Generation IV, then got Sacred Fire in X/Y (it still irks me, even if it is related to Ho-oh).
I mostly like Entei because of its volcanic colours, which are refined to a fault, and majestic figure making it one of my favourite legendary Pokémon of all time. Raikou looks quite neat too thanks to its simpler, almost catlike attitude.
You know, I'm sort of stuck between Raikou and Suicune. I voted for the latter though because he's more iconic due to Crystal, so that edges him up a bit. Entei had a huge role in the 3rd Pokemon movie so I like him too, just not as much.