4ll Commands
Since it's at a point where you're still in the Shinra headquarters for the first time and making your escape, I'd like to avoid being MP-reliant, since you're likely a low-ish level (unless you've been building); so I'd prefer the stamina and utility of Command Materia. Most likely, quadra-cut (x4 cut) and w-item, because that would make you quite unstoppable by anything in the Shinra HQ, as you should have acquired a hi-potion by this point. Quadra-cut turns even a weakling into a damaging machine, while W-Item allows you to both rapidly heal and clone items; allowing you to copy an Elixer which you should have acquired by this point.
Additionally, if we were to restrict it to base-level materia (ruling out stuff like Ultima which requires a level to even cast, or E. Skill which requires you to have encountered and survived your spells) then I would still go with double-cut/w-item for pretty much the same reasons.
W-item would also allow you a wide variety of useful tricks due to cloning items; such as being able to replicate your items such as
deadly waste or
ghost hands which you probably acquired before now (if not ghost hands due to lack of a
Steal materia, then deadly waste falls off enemies in the #5 reactor); and the ability to drop Bio 2 on all enemies via an item is pretty nice.
In fact, you could probably comfortably take the entire game on using 1 character and x2 cut and w-item, if you make good use of combat-use items and the ability to replicate them using w-item.
Hm, as of writing this, I'm tempted to see if I can find some Gameshark goodies to nab some late-game materia and play with it in the early portions of the game. It'd be fun to make a command-based party who sports stuff like manipulate and w-item early on.