Best type of metal for turntable platform?

"There's crawl space under my floor."

Then...

When weather permits crawl in and use basic/inexpenisve items to support the floor under your TT.

You would need to lift the joists (scissor jack form your car?) maybe half an inch, or so, and then block/support them with cinder blocks/and perhaps suitable non-compliant shims allowing gravity do it's job once the joists are lowered.

Hate crawl spaces myself, but once done (depending on the lower/ground surface) the remedy should last years, if not decades.

I used to place my TT's in closets (including unused Murphy bed closets) in old rental units due to to the added bracing/structure those specific areas afforded.

Curent TT is also in a closet (along with all the electronics), but for other reasons.

I now use specialty shelving, upturned brass spikes, et cetera, however my floor is quite stable (aside from the occasional earthquake and/or Air Force afterburner flyovers/drills).

A stable floor is the main goal, if it can be accomplished within reason/budget. 

DeKay

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.