To make a well-flavoured liqueur with a good kick you need good quality strong vodka (or other spirit where specified). Normally, we use Smirnoff Blue as this is 45% as opposed to the standard 40%.
It is also recommended to use Polish pure spirit - an incredibly strong spirit ranging from 79%-90% proof. Obviously, this is dangerous stuff in the wrong hands (or mouths!) and you must never drink it neat. Seriously - this stuff could kill you!
As it is such a potent product you may not see it on the shelves of your off licence but, if you ask, they may well stock it and store it under lock and key. If you don't want to go through the embarrassing rigmarole of explaining how you're not an alcoholic but need it for home-made liqueurs then I recommend buying online!
It's not cheap but worth using sparingly in your liqueurs.