It has to be the Roll d100 and add something then prey for an Open Ended.
That is the basic core mechanic of RM in all its flavours and it is just so flexible that it can be applied to just about every situation, skilled or not and has that little bit of excitement about the open ended roll.
I really try and avoid D100 rolls that are not OE as they seem a bit flat to me, I think the only real use for them is stats and criticals and I like point buy stats. It is the magic 66 result that adds the same sort of excitement to the critical roll.
So it is the simplicity of the Rolemaster core mechanic that I like the most and it is also the one that is chartless/table-less and the one we use the most.