I haven't had a chance to use these in any recent games but if I recall correctly (back 20+ years ago) the Tarantula worked quite well in its role of sniping out vehicles and such. The Rapier on the other hand looks pretty cool but if I recall proved disappointing in practice, with the crew easily getting killed. The Thudd Gun I have little recollection of and I need to sort through my White Dwarfs and find the rules for it.
Tarantula:
Rapier:
I think something like the Ontos is what I had in mind when deploying a Rapier
Thudd Gun