against transports - autocannons lots of autocannons
against light/medium tanks - ML/Lance weapon
Heavy Tanks - lascannon/Lance weapon.
it usually comes down to what army i plan on playing what i bring.
Against anything av12 or lower, autocannons, scatterlasers, multilasers.
Against av13-14 tanks: melta if you want them to die (or railguns!. Otherwise.. yeah, missiles, sideshots with the above weapons, lance and lascannons, etc.
Most medium tanks aren't going to care that your weapon is "lance". Preds might, but only if you can't get to the side armor, and the rest of them that come to mind are av12 all around.