protilers

6mm and 8mm a must have if you are going to fit soapholders etc.

i find 16mm (personally) to tight for 15mm copper pipe......i choose 18mm for that.
i also use my 27mm and my 38mm alot.....showerheads,recessed shower valves etc....

my advice is invest in quality....it will last.....for many many holes,
but because the are so expensive buy them as you need them....if your next job only requires the 18mm for rad pipes hanging out the wall....just by the 18mm,all these useful things will end up in your toolbox as you go along........
along with some other stange impulse buys.....like my ultrasonic measurer (useless on a good day).......

good luck!