Putty For filling in cracks?

Squadron putty, Tamiya putty, Aves Apoxie Sculpt, Aves Apoxie Paste, AB putty, Magic Sculpt, all work well for me and are dependent on the application. They all sand, file and carve well and are easily applied and perform several other conversion/sculpting and enhancement duties.

 i use ambroid proweld on my joints and never have to fill seams/joints. i sand the plastic (assuming that's what u'r talking about) joints with a small file then put some proweld on it and press tight...no more filling and sanding!!!

I use Bondo Red Glazing Putty for 80% of my filling-work (Get it at Wal-Mart or auto supply stores) and the rest is done with Testor's white putty.  I like Testor's on figures because it can be thinned with alcohol, so a finger dipped in rubbing alcohol will work for smoothing down and pushing it int the cracks & gaps.  Un-thinned, it builds up nicely for fixing the big gaps left from modifying figure's poses, like the tanker below...