Bit of a strange question. A canvas is just one small component of the UMG which, in turn, is a visual editor for Slate.
What do you mean by best customisation? UMG has its limits, once/if you hit them, you can start messing about with Slate.
For something very simplistic, the HUD class might be enough; UMG can be fairly powerful if you can live with its quirks; Slate documentation was incomplete last time I checked. Haven’t checked in a while, though.