The ENIAC was the first successful electronic digital computer. The Fiftieth Anniversary of ENIAC is fast approaching.
The IBM SSEC is something I know nothing about.
The IBM 360 was a revolutionary advance in computer system architecture, enabling a family of computers covering a wide range of price and performance.
The LGP30 was built by Litton General Precision in the mid 1950's. It was implemented with vacuum tubes and drum memory. It used a Flexowriter for I/O. The instructions had three addresses, two for the operands and one for the next instruction.
. Digital Equipment Corporation's first computer was the PDP-1.
Spacewar is the first video game and was written by Steve "Slug" Russell at MIT in 1960-61.
The PDP-6 was DEC's first 36-bit computer.
The PDP-8 was the world's first minicomputer. It was priced at the amazingly low price of $20,000.00.
The DEC PDP-11 was a wildly successful minicomputer.
The DEC VAX 11/780 brought mainframe capability to the minicomputer market.
You may obtain more historical information from the Charles Babbage Institute. Also see Paul Pierce's Computer Collection. J.A.N. Lee maintains an unorganized History of Computing. Arnold G. Reinhold has some articles on his own personal history with early computers. Yahoo maintains more comprehensive information on the History of Computers and the Internet. See also the Big Computer List (local copy).