OP, I think your question is too broad to answer with just one answer.

For conventional combat employing unit formations I'll offer a few of the best. The USMC, the US Army Rangers, the Russian Naval Infantry, and the British Royal Marines.

For non-conventional special operations warfare it is hard to come up with a better answer than the US Navy SEALS, though I'm sure there are probably a hald dozen other specops units that are in the same league.

Most powerful? That could mean a few things, but I'll assume it means what organization brings the most firepower indigenously within a brigade or regiment size force. I'd offer up a cavalry regiment of the US Army or Marines. It comes with mechanized infantry riding Bradleys, Armor riding Abrams, automatic grenade launchers, mortars, MLRS missile artillery, and Apache helicopters. All trained to work together as a combined unit. I'd hate to be on the other side of that regiment when it comes looking for a fight.