The highest terrain that I've come across is y=189 in my own modded version (I don't play vanilla past 1.6.4, when terrain was limited to below y=128), with the highest trees being higher (I only measure the terrain itself):
The highest terrain I've found in Survival is y=184, one of at least 13 points reaching y=128 or higher that I found in "TMCWv4" (the seed in my mod of the same name/version); I only counted the highest peak within a given biome if there was more than one and as mentioned above things like trees do not count:
Here are screenshots and renderings of many of the points listed:
This is a scaled-down rendering of the entire world, showing the great variety in biomes and terrain I found; the world is about 3200 blocks across. There are also more high peaks visible which are not shown below, such as near the top-right corner:
Both the Rocky Mountains and Ice Mountains can be seen here:
Both the Flower Forest and desert mountain can be seen here:
Also, it is theoretically possible to find a "mega tree" that reaches the height limit as they are up to 64 blocks tall and the highest possible terrain in TMCW is y=192.