Gonna post this from this thread which also has a similar discussion you'll want to look at it:
http://www.dakkadakka.com/dakkaforum/posts/list/549952.page
Basically you want to break down the role of the tactical squad in your army. Also this doesnt take into account all of the army options like scout/hit and run,
which might make stuff like the rhino less necessary (though they are good for protectiona nd overall mobility if they survive)
"Home" squads: these squads are those that will want to sit on an objective and, as such, are great for lascannons/
PCs and also man quad guns and the like.
Furthermore you really dont need more than 5 guys here, so no need to spend points on special weapons either, keep them small,
lean and get 1-2 of them instead.
Midground squads: these are the squads that you want to head out and objective capture. As such they will want a rhino to keep their mobility.
As a result of all this the best heavy weapon may just be... none at all. Given that they are expected to be on the move almost every turn
it's somewhat useless giving them something they wont be able to use more than once unless you string them along.
However if you are set on it then a
HB is the way to go here, because you at least 3 good snapshots off you can use to put down what will be in your way: other infantry.
The best special weapon here is clearly the plasmagun, as it gives the the ability to start dealing with heavier hitters.
Close squads: Finally the closeup ones, the guys u'll be outflanking with or podding in to get linebreaker early and hopefully survive long enough to do something.
The special weapons of choice here are clearly either the meltagun or flamer, depending on the role they need to fill (anti mech or anti infantry), and the heavy weapon
therefore would be the
MM or
HB (since sadly we dont get the heavy flamer as an option) respectivly. Here again though consider not having a heavy weapon at all
It all depends on if they are meant to survive more than one turn