Tacticals:
10 man, plasma, combi-plasma, multi-melta (24" kill range)
Add rhino/razorback if you have other armour for saturation
Scouts:
Snipers - add a
ML for shooting at flyers
Don't bother though,
IG are cheaper & affect a similar job
Bikes:
5 strong, double plasma/melta, combi-same, multi melta
Allies:
Imperial guard platoon for objective camping
MOAR Space marines
Firewarriors/eldar/whatever - keep cheap, you don't want their troops
=][= - I don't need no stinking troops
The troops need to "fit in" with your other elements. Having tacticals on foot in an armour heavy list is asking for trouble for example.