When I was a kid (and still now) I loved Terminator 2. The special effects at the time were the best we had seen yet. The way the T-1000 morphed in and out of liquid metal blew my mind. Now days it would probably be done 100% with CGI, but when this movie was made CGI was still in it's infancy and they had to use a lot of puppetry. T2 had a lot of CGI firsts though, including being the first movie to have realistic human movements on a CGI character, the first use of a PC to create major movie 3D effects, and many other firsts. Check out this BTS video on how they made some of the molds and puppets for T2.
Terminator 2 T-1000 Special Effects BTS