The San Jacinto Monument is listed on Obelisk#20th century.  The question of it being an obelisk was raised in 2006 and got no replies; while this question also from 2006 got one (negative) reply.  I would say, there is for now no actual consensus.  However, if San Jacinto was ruled to not be an obelisk, the next tallest could be the Juche Tower in Pyongyang, North Korea (which is about 2 ft taller than the Washington Monument).  It has four sides but then again you could argue the top is still too ornate for it to be an obelisk.