All of those listed are great, but I'm pretty sure Fatal Fury is the first franchise I played by SNK. So I picked that. I played the SNES Fatal Fury first.
I'm surprised to see The Last Blade on the list, especially over Art of Fighting, which was a rather formative set of titles for the company -- good call!
(Because it's my pick and I was gonna write it in anyway if the option wasn't there.)
Behind that,
#2) Samurai Shodown (For SS4:S and SS6 in particular, though 2 and 5:S are also great)
#3) Fatal Fury (For Real Bout 2 and Mark of the Wolves)
#4) King of Fighters (For 2K2:UM especially, and 99 and 2k as well)
#5) SNK Vs. Capcom (For Card Fighter's Clash -- my favorite card based videogame.)
#6) Metal Slug (I love the whole series, but I can't sit through any of them anymore. ; -; )
I don't enjoy Ikari Warriors, so I won't bother placing it.
I don't have a favorite. It's just a shame they are pretty much only known fo rKOF these days. As good as KOF can be, Fatal Fury and Samurai Shodown were great.