Criteria for Rating a Locomotive Sighting

[Trainspotter]

Like most train buffs I have the urge to collect, tick off, record, to
have the full set, etc. when it comes to observing trains and
locomotives. And while my ultimate goal is to see as many trains as I
can, there is joy to in simply spying a rare one.

I was wondering what criteria other diesel and train-spotters regarded
as most important when rating or ranking a particular locomotive
sighting.

My criteria would include:

- Rarity, one-of-a-kind, or unique locomotive
- Distance travelled to see the particular unit
- Weather or conditions encountered
- Distance the unit is away from home territory or its usual range
- Historical value (oldest, last of class built, first of class built,
etc)
- A milestone (first of class ever spotted, last needed to complete a
set/class, etc
- Personal favourite

Any thoughts on what other criteria train-watchers utilize to "rate"
their particular sightings?