Think of 4 distinct groupings . . .
1. Race Skis (think real FIS race skis, no rocker, designed for racers and race courses)
iDHrd, iSGrd, iGSrd, iSLrd (i=intelligence,rd=race dept)
Head also offers an iSL and an iSpeed, they are non FIS radius compliant.
2. Performance (similar to race skis but with a slightly softer flex and shovel rocker)
iSupershape (Speed, Magnum, Rally, Titan)
3. Allride (about 80-108 mm at the waist)
Raw Instinct, Power Instinct, Monster 88, Monster 98, Monster 108
4. Freeski (AKA Flight Series-think fat, rockered twins).
Venturi 95, Collective 105, Cyclic 115, Turbine 125
If you're a racer, the choices are race skis.
If you're a big mountain skier, the choices are Monsters or Flight series.
Monsters are directional and powerful with a straighter tail.
Collective/Cyclic/Turbine are more playful, maneuverable twins.
If you're looking for a technical* ski look at the iSupershape or Instinct series.
ISupershapes (waist range 66-80).
ISupershape Speed (66)-this is a SL ski with shovel rocker. At 66 under foot it's designed to turn all the time. It's amazing on groomers and the shovel rocker makes it slightly more versatile then a pure race SL ski.
ISupershape Magnum (72)--If there is any obvious overlap, it's between the Magnum and Rally. It's a personal choice mostly. Most of my instructor and coach friends buy from this part of the line. Magnum and Rally are the work horses. Magnum (72) is a hair quicker turn to turn, but the Rally (76) is a hair more versatile off groomers. They are both similar with the Magnum being a bit more popular in the East Coast and the Rally being a bit more popular on the West Coast. Both are incredible.
iSupershape Titan (80)--this is the widest ski in the group and it's amazing too. This ski appeals to somebody who wants as much edge hold as possible in a ski which is narrow enough to be a good carver but definitely wide enough to work all over the mountain.
I need to throw one curve ball in here. If you look at the Race line, you will find another ski called the ISpeed. It is a different ski then the ISupershape Speed. The iSpeed is a GS ski with an 18m radius. This ski should also be considered if you want a GS sidecut vs SL which translates into a longer turn radius.
In summary of the above category, go iSupershape Speed for pure carving, or go iSupershape Titan for all mtn carving (with race like performance). The Magnum and Rally split the difference, Magnum being a tiny bit quicker edge to edge, Rally (while still lightning quick) is a tiny bit wider and a slightly better West Coast choice.
Power Instinct and Raw Instinct. Instincts are less expensive then the ISupershape skis and have a slightly longer turn radius making them a bit more compliant for all mountain skiing. The Raw is 78 under foot and the Power is 82 under foot. When I say less expensive, don't think less performance. Instincts don't utilize the intelligence electronics, thus making them more affordable. My wife got a pair of the Power Instincts in a 163 and is claiming them as her favorite skis of all time for teaching and technical skiing. She also owns Magnums. If it's rock hard conditions or she's mostly skiing groomed terrain-she reaches for the Magnum. If she's going to ski the whole mountain (non pow day) she grabs her Power Instincts.
One last thought. As a former instructor I've long had a personal opinion that skis undergo a definite change in feel and precision somewhere between 75-80 under foot.On hard snow/groomed terrain sub 78 and narrower is ideal and gives the skier a mechanical advantage, better chance at edge hold, more precision and reduced chatter. At 80 or wider under foot, a ski looses some of the precision BUT has more surface area and plays better all over the mountain.
My point is the dividing line between all mountain and technical starts somewhere at 75-80 under foot.
Head offers a ton of choices in this area.
Looking for hard snow compliance? Go iSupershape and pick your width.
If you ski more all mountain look at Instinct.
If you ski big mtn go Monster.
If you like a loose skiing fat ski go Flight series.
I hope this helps.
* My definition of a technical skier is as follows.
You ski at an area which only has groomed terrain or it's a hard snow kind of day.
You work on your skiing trying to emulate Lindsey Vonn, Ted Ligety, Anna Fenninger, Lara Gut, Aksel Lund Svindal .
You are an instructor and know how to ski it all with a narrow ski.
You are a race coach.
You flat out enjoy the precision of narrow ski.