Apoxy-Sculpt (…) From what Jakko says, it may be superior.
Milliput is one of those things that a great number of British modellers swear by, and I (and probably a lot of non-Brits) mostly swear at. I used it for quite a long time mainly for want of anything else, and my chief complaint about it is that it's so hard —
before mixing, I mean. It's very difficult to knead the two parts together for all but the superfine varieties, and back when I did use it, the only superfine available was white. Yes, that meant you had to knead a white part and an off-white part together and
hope you got it mixed well enough.
I won't claim to have very extensive experience with epoxy putties, but over the years I've used a number. The very first was Verlinden's, which was sticky but kneaded together easily and generally worked well. Then I was forced to use Milliput, which was a big frustration compared to what I was used to, as I explained above. After too many years of using that (just not very often

) someone brought a packet of Sylmasta A+B from the UK for me, and that was just
so much better — kneading it together is not too much work, and once mixed it feels (and looks) like soft caramel.
After those I've also tried Magic Sculp, which is a little less soft than Sylmasta A+B but very good to use too, and Green Stuff, which I'm not a fan of. It's too rubbery, making it hard to knead unless it's warm, like from your hands when kneading it, but then when you go and work it it cools down and becomes hard to shape again. These days I turn to the A+B when I need to make larger things, like sandbags on tanks, and Magic Sculp for smaller, like figure conversions.