Weird reloc are the best by far. Generally because they aren't used... and if they aren't used, chances are the alien team doesn't have too much experiance dealing with them.. Which gives the marines an edge.
The ones commonly thought of as "the best" probably aren't. I mean, most are double nodes which are near impossible to properly defend or areas near or in hives, which are often sided for the aliens as they are on the alien side of the map.
Reloc 1: Atmos on bast. Lets face it, everyone relocates there at least once. Aliens are used to it and probably have a fairly good idea of how to go about dealing with it by now. Marines set up base, but are unable to take Feed properly. They get pwned. Game time 15 minutes.
Reloc 2: The vents west(?) of MS on bast, going towards Feedwater. They aren't common for a marine relocation, yet I have seen marines with out hmgs, sg or pretty much anything short of an armslab put up a very good fight spanning from 30 to 60 minutes in there against 3 hive aliens.. and it was a last chance relocation. There was a good 30 minutes of fighting before hand.
Now, in both situations the marines lost. But, they were able to survive a lot longer in the vents than in Atmos, although the vents are not very close to many res nodes, they still served for great natural defence against the aliens. Most of the time with the aliens forced to come through long, narrow openingings to get to the marine base.