Bolt pistol, chain sword and a jump pack is all a Marine should ever need.

warscythes are used by necrons

the item you are after is a manreaper

For ranged fight: heavy bolter, "reaper" autocannon, combi-bolter.
For melee: claws, power sword and of course chainsword.

Ranged: either bolter/bolt pistol or flamer (nothing to fancy)
melee: Power sword or Axe (nothing better to chop up the enemies of the Imperium)

Can't go wrong with a Melta, like a much better Plasma Gun if you think about it.

Graviton gun. Talk about OP.

Lightning Claws.

However, if range is needed (phaw!) then give me a Plasma Pistol and a Power Sword. Storm Bolter if I'm wearing Terminator Armor.

Well, a trusty Bolter of course.

And in the other hand a Power Axe, or rather a Frost Axe. Nothing like Kraken Teeth shredding armour and innards.

Thunder hammer is the best.... but pair of hammers are beter.

The tried & tested bolt pistol/plasma pistol + power sword.