I've rigged up one that's partially based on Erky's and partially my own experimentation before reading most of this thread. I had gotten DF to make a ton of small mountain ranges and single-tile mountains in order to make finding "perfect" sites more likely - I then used several things from his to improve it further, such as the higher amounts of rivers and volcanos (I hadn't quite figured out how to make volcanos more common without getting infinite rejection loops).
This seems to result in extremely small dwarf civs, however. On my first map, all three dwarf civs were still only one tile in size in the year ~330 or so (which is when 80% of the megabeasts had died, and 50% of them died by the year 50, with checking beginning in the year 10).
The goal of this was to make sites like this more likely: I found this after looking through only about half of my first map:
A mountain tile with a 6x6 embarkable area containing surface water, trees, plants, sand, all four kinds of stone, flux, and all the mountain/rock tiles fit into a 7x7 area (guaranteeing all manner of Hidden Fun Stuff, if embarked to that area, even if you didn't use the prospector to find them prior to embarking). The only thing missing was surface magma, but the prospector shows that it does have underground magma (one source in the 7x7 area, luckily). (It's also untamed wilds)
I don't know if it's common, but the first map I generated with it also generated without any failures, too (You'd think it COULD have failures, since I left some limits enabled).
World gen code:
[WORLD_GEN]
[TITLE:SL_IDEALSITE1]
[DIM:257:257]
[END_YEAR:10000]
[BEAST_END_YEAR:10:80]
[END_YEAR:10000]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:250:305:800:800]
[RAINFALL:0:100:400:400]
[TEMPERATURE:0:100:200:200]
[DRAINAGE:0:100:400:400]
[VOLCANISM:60:100:1600:1600]
[SAVAGERY:40:100:800:800]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[GOOD_SQ_COUNTS:100:1000:2000]
[EVIL_SQ_COUNTS:100:1000:2000]
[PEAK_NUMBER_MIN:0]
[OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:200]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:25]
[RIVER_MINS:800:800]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:800]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:0]
[TOTAL_CIV_NUMBER:50]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:8256:16512:8256]
[DRAINAGE_RANGES:8256:16512:8256]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]