For even the stiffest tyres
Park Tools TL4C - Tyre Lever - Set Of 2
Great levers that are easy to find in the tool box
These Soma tyre levers have a steel shank running right through their plastic body to provide extra strength, but they're still thin enough to slip into tight gaps and get tyres off.
City and mountain bike tyres go on and off by hand as easy as Tupperware tops. Narrower road or touring tyres don't. Sure, with decent tyre levers, a bit of thumb strain and a dusting of talc, anything's do-able. But easy? Not always.