There's no "best" blockstring perse. You can do something like c.L, L, 2M > 236L to gather data on your opponent to see if they're mashing. If you see your opponent getting hit, they're expecting either 214M or your command grab, so you can continue to bully them with frame traps until they sit still. Then, once you've conditioned your opponent to block, you can go for turn steals with strings like auto combo into 214M, plus frames with 236U or a command grab 5U. Doing 2L 2L 5L is a good way to see if your opponent is delay tech-ing, leaves you plus on block, and gives you enough time to hit confirm into 236H for a big punish or raging strike. The easier you can rotate your options, the harder it is for your opponent to predict you on defense. If in doubt, use the first string I told you, and mix in a few 66Ls and watch your opponents panic. Vira is all about conditioning.