I always focus on Vigor, Speed and M. Power. I usually get all characters to around 110 Vigor and M. Power and at least 40 in Speed. I've never seen the use of raising Stamina.

The highest I ever got for M. Power was the max of 128. That was with Terra but she took a hit in Vigor and only got 109 for Vigor. The best I ever focused on for Vigor was Sabin, again with 128 but that made his M. Power drop to 105. The best in Speed of course was Locke with a Speed of 60 and I evened out the other two stats at 107.

My most well balanced character is Shadow who has 115 for both Vigor and M. Power and 49 for Speed.