Best Time?

Scotland is quite a large place, and what is available varies considerably from place to place.

And what does "BEST" mean to you? Before kids are out of school? After kids are out of school? For winter snow or a chance at the Northern Lights? For the midnight sun?

I'm throwing darts really because your question doesn't give me much to get my teeth into.

May/June because of the long hours of daylight.

Do you want five hours per day of daylight or 20?

We liked mid to late May. One of the dryer (less precipitation) months, lengthening daylight hours with maximum sunshie, mild temperatures and low midge season and fewer tourist (than peak summer).

https://www.metoffice.gov.uk/climate/uk/regional-climates/ws

The number of hours of bright sunshine is controlled by the length of
day and by cloudiness. In general, December is the dullest month and
May or June the sunniest.

Scotland tends to remain under their influence for much of the summer
too. Autumn and early winter are the wettest seasons, especially from
October to January, and spring and early summer is normally the
driest part of the year, especially from April to June.

https://www.smidgeup.com/midges/midge-season/

Midges begin to emerge during May (usually mid-late May). These are
typically the non-biting boys. When the biting females arrive in
early June the season truly kicks off. Traditionally, our largest
catches (in Midge Forecast traps) have always been the first week of
June. Midge season then runs until early/late September.

Haven't been yet, but we are going in May - mostly because I want to avoid the midges - I hate black flies and mosquitoes with a passion, and don't want to spend my whole vacation either hiding indoors, or swinging my arms about my head and cursing under my breath as I do at home (I'm sure my neighbours wonder about me). Glad to see others recommend May!

Northern Summer, between the equinoxes. Also that is when places will be open.