2 was the best for actual fighting gameplay, 3 had the best outside content (good campaigns, extra modes, and create-a-fighter with actual unique movesets), IMO. 4 just felt underwhelming (they gutted pretty much everything but a brutally short story mode and arcade ladder and then PVP) and had the real most broken guest characters (the Star Wars characters). 5 is the "black sheep" of the franchise but I actually still enjoyed the short time I had with it, I liked the new characters that weren't copies/descendants of the old cast. 6 felt mid to me, don't have much to say about it since I played it even less than 5.