Hi DVX
I'm going bend the rules, because I don't think there's always a single "best" in every category!
Instead, I'll say what I'm using at the moment and, where applicable, what I want to try out.
1. Filler. This is a classic "horses for courses" category - you just have to look at Holdfast's thread to see the range of views. I use 2 main fillers:
a. Superglue (CA) - it dries quickly (esp. with an accelerator) and feathers in beautifully,
b. Vallejo 400 Acrylic Putty - this is great for small jobs where you don't want to sand... just wipe off the excess with a cotton-bud dipped in nail varnish remover. It won't attack plastic - but will remove paint!
c. New to me is Superglue + Talcum Powder (as recommended by ZoomieE7 and Scoccia). I've only done a test but this seems amazing! It dries quickly and sands easily.
2. Decal solution.
a. I use Solvaset or SuperSol and dilute as neccesary. Nothing wrong any of the others - its just what's on the the shelf.
b. If all else fails, I float a decal in Klear (Future).
3. Matt Varnish.
a. I'm still using an old bottle of Micro Flat.
b. I've got some Vallejo matt varnish, but haven't tried it yet - anyone got any views?
4. Primer.
a. Humbol No1 mixed with cellulose (laquer) thinners to make a "hot" mix. Goes on quick and smooth.
b. I want to master Halford spray-can primers 'cause the rest of the world can be wrong!
c. Alclad primer. See the new Tamiya magazine. I haven't found any yet, but I want to try it...
Sorry to be so long-winded
All the best
Rowan