Where you are will determine the best product that is available to you (postal services are a bit wary of transporting corrosive/toxic/flammable materials - they tend to be concerned about their employees melting/dying/bursting into flames or suing them for any of the above).
For UK over the counter, I use Tamiya extra thin cement for Rubicon kits (they have a high ABS content and many liquid cements will not touch it).
I use Humbrol liquid poly in general, I have a "thickened" pot which has had bits of sprue added for minor filling and surface texturing.
A combination of the two (some slightly thickened Humbrol will tack a part in place, then fix it by running Tamiya extra thin into the joint by capillary action).
For the more chemically minded, you should be able to buy this at model railway shows.
https://slatersplastikard.com/plastikard/mekpak.php