Beefy assault marines
I have found that the more upgrades on assault marines makes people want to kill them quickly, but the beauty of assault marines is when they are vanilla with meltas (bolt pistol, chainsword) with more bodies, people ignore them because of bigger more terrifying units on the board, like terminators or HQ squads. But they make enemies fold really quickly with weight of dice regardless of the unit (i.e. terminator 2+ save is nice but try making it against 31 times at once).
GW sells a sternguard weapons upgrade kit that comes with combi weapons, powerclaws, flamers etc. that way you get a few spare weapons as well for upgrades on other units if you want.