I think the way the possibilities for AMMs have changed may be a big driver in new missile regimes. So I want to break down those options.
The Traditional
Single warhead, no fancy features. Can take advantage of fractional warheads to eke out a little more performance than before, but likely to lose a lot of accuracy from not having agility. I expect these to be obsolete except maybe at low tech levels. But they will have the best raw stats in a way since they aren't spending on anything else.
Flak
Multiple warhead, otherwise traditional. My guess is this will be the standard against which more exotic options are measured. With multiple warheads it's conceivable to use more than 1 MSP on these, but probably not useful unless you're concerned with extending their range through fuel efficiency.
Smart
Shelling out for Active Terminal Guidance. At high tech levels, this likely benefits even 1 MSP AMMs and almost certainly benefits any larger ones. Check how the fraction of engine size it eats up compares to the accuracy multiplier.
(ECCM I'm not sure about. Antimissile ECCM is radically different from anti-ship ECCM: it's worthless unless the ECCM is superior to the missile's ECM tech, while for ships it's the other way around. There are definitely scenarios where it would pay off but I suspect they're rare.)
Excessively Smart
Put retargeting on an AMM. At a full half MSP, this is an enormous sacrifice of space, especially if you're trying for a 1 MSP weapon. On the other hand, if you can outrun the target even a little bit (and intercept more than about a minute from impact), you get one hit for one shot, and have no need for any of the other enhancements. Unlike ASMs, a retargeting AMM doesn't risk repeated passes through point defense fire! These missiles are the ones I want to dig into, because they're probably not always the answer but they do do a lot to constrain the viable ASM space. A simple offensive cruise missile is at risk of a disastrous 1:1 exchange rate.
One way to survive excessively smart AMMs is decoys. Note, the math here is surprisingly harsh to the decoys - A missile with N decoys has an expected hits to kill of N/2+1 (assuming no ECCM). That means you're using a full MSP of decoys per excessively smart AMM!
The other counter is to be too fast for them. A 1 MSP AMM with retargeting is going to lose a lot of performance. A larger AMM does better there but bleeds against decoys. OTOH outracing an AMM is tricky for a long-range offensive missile, since they need to carry fuel or limit engine boost to achieve standoff range. This might be a big advantage for staged missiles. (And short-range torpedoes.)