1. At least 5-6 stages in the high mountains:
And by high mountains, I mean mountains in plural. A stage with only lower/easier climbs before a MTF to for example Pratonevoso or Oropa would not be sufficient to be included in this category. The stages should also be of different type (MTF, descent finish, etc) and distributed throughout the GT (although this is more elaborated in point 4). In the Giro, I would for example rated the mountains 10 if we had 2 big MTFs like a double Passo Lanciano in the first week and Monte Bondone the last week, a descent finish from a big mountain like Monte Grappa and two of this stages with big climb/smaller climb combo like Finestre-Sestriere and Fedaia-Pordoi. Especially in the Giro there should be at least two of the last kind of stages for a perfect rating. This is the most important point, I would deduct more points here than any of the other if the mountain stages were too easy and/or too similar.
2. Maximum number of (pure) sprinter stages:
Flat or more or less flat stages which ends in a big mass sprint should be limited. No more than 5 would be preferable. You could have a couple of extra which could end a in a sprint with a reduced peloton, but these should also be limited. If you had 7 almost flat stages and 3-4 more that could end in a sprint with a reduced peloton, I would have a hard time rating the GT a perfect 10.
3. At least 2 medium mountain/hilly stages relevant for the GC:
High mountains are good, but medium mountains should neither be ignored. And by relevant for the GC I mean something more than a Muro de Guardigrele finish or a similar medium mountain finish as Super Besse. The Torino stage next year seems like a really good medium mountain stage. And a hilly stage similar to those Tirreno stages that has prompted a lot of actions in the last few years. There should also be more hilly/medium mountain stages, but as stated at least 2 potentially very relevant for the GC.
4. A "correct" sequence of different type of stages:
Neither of the stages mentioned above would be top notch if the sequence of the stages were really bad. And of course the route should not be not backloaded. At least one of the high mountain stages and one of the medium mountain/hilly stages should be in week 1. And two or three more of these in week two. One should also avoid packing the high mountain stages with 3 or more tough stages in a row, and where the very toughest stage is the last. And avoid having big mountain stages the day before an ITT.
5. Something "extra" or original:
For a perfect route, we also need something extra. It could be a sterrato stage like the stages to Montalcino. Or a monster 250 km medium mountain stage with a load of climbs and a strategically placed tough climb in a suitable distance from the stage finish. Or something brand new in the mountains. Doing Col de la Loze, descending past Courchevel and climbing fairly easy section to Pralognan de Vanoise would be much more original than just paving the last few km from Meribel to Col de la Loze. But this point is less significant. If all the other requirements listed were fulfilled, I woudn't withdraw much without this point.
6. A suitable amount of ITT
This is certainly more important for some in this forum. I would say a suitable amount, given all the other listed points here were fulfilled, is perhaps 70-80 km. But if rest of the route was perfect, I wouldn't withdraw much if the number of ITT were signicantly lower or higher. Maybe I would have rated 9,5 if it were 30 km of ITT or 120 km of ITT, but the rest was more or less flawless.
Just remembered this thread I started last year. And supplementing with some additional thoughts:
TTs:
Maximum two stages of ITTs/MTTs and no TTT.
Total length should be no more than 80-90 km.
MTTs should be more gradual climbs of 5-6 % and not steep ramps like Kronplatz or Lussari. Best MTT profile ever was the Briancon-Sestriere stage.
Mountain stages:
4 big/high mountain stages is absolutely minimum. 5 or 6 is preferable if not "replaced" by really big medium mountain/hilly stages.
2 high MTFs is sufficient (in the Giro). Like Grappa, Siusi, Bondone, Montecampione, etc. in the north, and perhaps one Appennine MTF (Montevergine is not enough!).
The rest of the big mountain stages should rather be descent finish, a short ramp at the end of a big mountain stage (like the fort in Briancon, Col Druscie, etc) or the typical big/small climb combo in Italy. In the Giro there should be at least 1 or 2 of the latter in each version.
And a couple of big/long medium mountain stages please! In the Giro one of these could be a murito stage.
Sequence:
The latest time for a ITT/MTT stage should be something like the first stage in the last week (stage 16).
The biggest mountain stages should not all come the last 5 days. Stage 14 or 15 and 17-19 would be suitable for at least two of the biggest mountain stages.
The first week (or typically 9 days) should contain more than a fairly easy MTF and a average tough medium mountain stage. At least one big mountain stage, one big medium mountain or big murito stage and one more stage relevant for the GC (ITT, sterrato/cobbles, a second big mountain/medium mountain stage).
The main problem with especially the Giro the later years is the sequence of the most important mountain stages and that there are too much focus on MTFs (at least after 2019). This leads to fewer stages with aggressive racing and shorter sections with aggressive racing.
In addition there is a problem that there only rarely is a stage other than ITTs that separates more than 10-15-20 seconds until stage 13-14 or something like that. It's almost unavoidable that the best racing happens within the last 8-9 days, but they could at least try to create stage that separates more before that.
In the Tour the main problem has been too easy and/or short mountain stages and far too heavy reliance on unipuerto finishes. And secondary that there are too few good medium mountain stages in Massif Central/Vosges/Jura. Maybe this year signified a change. Rumours for next year also seems promising.