It depends what you fly. Whilst it is best practice to assume that if you are in a single engined aeroplane, the engine will fail at some random point on the current or next flight, the reality is that the majority of modern light aircraft engines are extremely reliable and most pilots will never experience an engine failure. (I'll admit to several including a couple of field landings, but much of my flying has been with uncertified engines and a fair bit of that test flying new homebuilts).
Unless an aeroplane has good enough glide capability to thermal, then a good glide ratio will only extend your options, you'll still be coming down eventually (and that's even true in a glider, albeit possibly much later). However, if you are looking for very good glide performance from a single engined aeroplane then the best will certainly be from a motorglider such as a G109b (or a microlight/motorglider hybrid such as a Chevvron). After that, it's likely to be the modern composite aeroplanes that are best such as the Diamond DA40 or the Europa.
For water crossings "hoping for the best" is not the best approach - it's more a combination of confirming likely engine reliability, carrying appropriate safety equipment, planning a route which minimises time over water and maximises rescue opportunities. For flights over mountains, the same applies - to be honest I'd rather have an engine failure low over the English Channel than low over the middle of Snowdonia.
G