Great thread, John.
I could say "I use such-and-such" but, seeing as I've only re-started in the hobby a few months back, it would (incorrectly) give the impression I actually know what I'm talking about - that I've tried lots of different stuff and settled on what I consider 'the best'.
Truth is, I've bought and tried out Vallejo Plastic Putty and Milliput. Both seem fine, in their own ways.
I use (there you go, I couldn't resist!) VPP for small gaps and Milliput for larger ones. VPP seems to get a bad press for being fragile and crumbly, but I've not found that too problematic, used in very small volumes. Milliput is great for building up scenic bases, in conjunction with cork sheet and/or Scots pine bark, followed by some sort of 'mud' and texture media.
I'd use Milliput for smaller gaps, too, but it's a nuisance having to make up batches, and I waste 98% of what I've made up, as I find it difficult to accurately mix tiny, tiny quantities. VPP is handy, straight out of the bottle.
But I'll be getting some Deluxe Materials Perfect Plastic Putty after reading this thread, and as I've generally found Deluxe Materials' products to be really good.