Plasma beam in Metroid...because it's just so fun watching your enemies burst into flames and quickly reduced to cinders.
Chemical thrower in Bioshock...you got your basic flamethrower functionality, with an electric discharge mode that's just phenomenal at stunning as well as zapping to death anything in front of you.
Ripper in Dead Space...frustrating because you need to train it on your enemies to keep inflicting damage, unlike the Flamethrower, but great at suppressing fire, as in it holds them back while simultaneously dealing damage.
SPNKr Rocket Launcher from Halo...it blows shit up.