The top of the line ambi safeties are the Wilson Bulletproof and the EGW.
Both are machined from bar stock, not cast like most all others.
Both are retained by a longer hammer pin. This is the most secure retention method of them all.
The type most seen around are copies of the original Armand Swenson type that has a paddle on the right half that fits under the grip to retain it in place.
These often fail to stay put and the rear of the right side safety moves up out of the frame and disengages from the left side.
Both Wilson and EGW use greatly improved methods of joining the two sides together. Most other brands are joined by a simple tongue and groove joint that tends to loosen or even break with use, and they tend to disengage easily.
Some of the better brands of the Swenson design are durable, but not nearly as secure as the Wilson or EGW.
Both are expensive, but are the best on the market.
The Wilson Bulletproof is available in blued or stainless, and in standard or wide paddle:
http://shopwilsoncombat.com/Ambidex...-Levers-Bullet-Proof-Blue/productinfo/192BBP/
The EGW is also available in blue or stainless.
http://www.egwguns.com/1911-parts/egw-hd-ambi-thumb-safety-machined-from-barstock-blue/
Long paddles can be shortened to be similar to the old Colt type ambi safety.