You've got the basics.

Fundamentally you need to know what priority the various projects/high level tasks have.

Then you need to understand what the "critical path" sub tasks and associated schedules are for the projects.

Simplistically you work on project priority 1 as long as there are any critical path items due.

Then you look at critical path items on priority 2, remembering to keep an eye on project 1 for any tasks that become critical path there.

And so on you go down your priority list.

However,...

This assumes you have a meaningful way of prioritizing projects/high level tasks, and that your work load does not exceed your capacity over the long term.

Ideally you have some management guidance on prioritization of the projects, failing that you need to work out your own prioritization.

For instance you'd hope priority correlates somewhat to impact on the company's profitability etc. but even this may not be entirely clear - for instance there may be conflicting demands on resources from projects that have short term impact V projects that have longer term impact - how do you pick which to prioritize?

For instance, I'm heavily involved in customer 'specials' where we modify our standard products to meet customer needs. I have to balance completing orders we've received and so impact profitability short term (i.e. this quarter or next) while also supporting quoting future orders which impact medium term profitability (i.e. next few quarters).

All the while I'm also working on longer term 'standard products' that while we may not have immediate orders hinging on them, will contribute to the medium & long term revenue more than most individual specials.

It can be tempting just to look at revenue $ tied to orders but that is usually overly simplistic, at a minimum should probably be considering gross profit on the order.

Realistically you also need to consider things like whether to work on the small $ order for a major repeat customer, or the bigger $ order for a customer that's likely to be a one off and similar.

Then there can be the politics factor, it may be bad to slip a small $ task if that task is a priority of some senior manager for reasons that may not be entirely clear to you...

Critical path is one aspect, as Kenat points out. Often, there are risks that are not immediately captured in a normal critical path analysis; these risks can impact both schedule and cost, and their priorities need to be factored by their risks, which may raise their priorities relative to critical paths. Sometimes, you can see these if you do a probabilistic critical path analysis, as the risks ought to be captured in the probable schedule variances.

TTFN (ta ta for now)
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I can see that both of you have real project management experience. I think you have summarized most of factors for prioritizing tasks. Thank you for sharing your experiences.