best way to make obsidian in minecraft?

for a nether portal you don't have to mine any of it. build a mould out of cobble, pour lava in where each frame block goes and tip water over it, and it sets right in place. no diamond pick needed.

water onto a lava source block. it has to be a source block — pour it over lava that's flowing downhill and you just get cobblestone, which is what most people are actually doing wrong. then diamond pickaxe or better to mine it. anything less and it still breaks, it just drops nothing at all and you've wasted the lava. it's slow even with diamond, roughly ten seconds a block, so that part's normal and not you doing something wrong.