2 Plasma Cannons and 2 Lascannons is my choice of equipment. I play against chaos a lot, so the Plasma Cannons help there while the Lascannons can do something at least, and for the tanks all the weapons work well. Unless it's a Land Raider. That's why I have the Lascannons. In my opinion it's the best if you face MEqs often, including the Inquisition.
Against Orks the Plasma Cannons do wonders, and the Lascannons will take out tanks like a hot knife through butter. Same with Tyranids, except the lascannons are used against Carnifexes and such, but the Plasma Cannons work well here as well.
Against Tau and Eldar (including Dark Eldar), it takes out everything. They have light armour and light tanks, and Wraithlords and Battlesuits aren't too bad a worry. Battlesuits are just shifty MEqs, and Wraithlords out up a good fight but in the end are just a rather large MEq with a higher toughness and an extra attack.
Of course with Imperial Guard it doesn't matter what you use - they die either way!
I only have 5 men in the squad. I hope to distract the enemy with much juicier targets, who are slowly advancing and if left unchecked will rip their army to shreds, (i.e. Assault Terminators in Land Raiders) however in games of more than 1250 pts 10 men are needed dearly, so I'm going to get the Combat Squad boc soon.