Best type of metal for turntable platform?

Soft lead is a good material for a platform.  Lead also works well when added to a well-damped platter to add mass.  VPI's best turntable platters were the 25 lbs acrylic-lead ones.    

Unfortunately, lead is considered unsafe and does not meet RoHS requirements.    

Ok.. yes.. as much as i said i can never handle my crawl space and would never go under there.. if its for the love of getting rid of vibration for the ultimate analog bliss.. im gonna do it! Im gonna lift this freaking house up. Do i move out as much furniture, etc., out of the room before lifting it? 

foot fall is when you walk up to the turntable and it starts skipping because the floor is bouncing under you.

A well isolated turntable will not be bothered by anything. This requires a suspension. If you really want to isolate an unsuspended turntable you get a MinusK platform. They are the best. But , if you get a properly suspended turntable you can put it on a foldable card table and it will not care. Examples are the SME turntables, Sota turntables, The Avid Actus,
The Kuzma Ref 2, Basis turntables, Dohmann turntables, Oracle turntables and probably a few I do not know about. You can put almost any turntable on a MinusK platform and get excellent isolation.

Don’t go crazy, but dekay has a reasonable idea. When actually leveling a floor, you have to know what your are doing, and do it fractions at a time, for a variety of reasons. All you want to do is reinforce, stabilize, and provide support at the joists along the area where your turntable is. Thus, if you use a jack, I would not raise the area more than a 1/4”, put a couple 2x4’s, or perhaps get some treated 4x4’s, stand them in place (on a 12x12 concrete block as a ‘footer’) and lower the joists back down upon the vertical 4x4 with the jack. Pretty easy actually, but depending how much crawl height you have, can be cramped, so scope everything out, precut the lumber, etc. There is no real need to make full height pilasters out of block for what you are doing and trying to achieve, that would be overkill.