What is the best term in (global) copywriting: "sticky tape", "tape", "scotch tape" or "sellotape"?

Adhesive tape seems perfectly fine.

Both Scotch tape and Sellotape (not cello) are proprietary brand names (owned by 3M and Henkel) and may not be recognized in countries where the tape was either not marketed or not marketed under that brand name. It seems 3M marketed it as Sticky Tape in Australia.

If it is important that your audience understand that the tape is transparent, then simply call it, as you wrote, transparent adhesive tape. However, I suspect that for most people, adhesive tape is almost automatically assumed to be transparent unless you explicitly state the opposite.

As TRomano points out, U.S. audiences may indeed understand adhesive tape to be (non-transparent) masking tape, so your best bet is probably to include transparent.

I'd go with "sticky tape", or "clear sticky tape". I don't think it' childish, and I think it's the generic name least likely to cause confusion.

Adhesive tape to me is a broad term that encompasses sticky tape, electrical tape, masking tape, duct tape, and perhaps some forms of medical tapes.

Try the different terms with google images and you'll see what I mean.

Cellophane tape is a generic term for that kind of tape.

In the US "cellophane tape" is the generic (non-trademark) term for clear sticky tape of the sort commonly used to fix paper items.

"Adhesive tape" is normally reserved for the (usually) white medical tape.

"Masking tape" is the term for a tape made from opaque paper (usually a light tan color, though often green or blue), where the tape is supposedly intended for use by painters to "mask off" areas not to be painted.

"Electrical tape" is the term for (usually black) plastic, rubber, or occasionally cloth tape of the sort intended for electrical wiring.

"Friction tape" is another term for cloth electrical tape.

"Duct tape" (more properly "duck tape") is a relatively wide (usually 2-3 inches) tape made from lightweight cloth ("duck") with a sort of thin plastic coating, with a fairly gooey adhesive. This is an inexpensive tape commonly used for sealing packages, temporary repairs, etc. (It, BTW, should never be used for sealing air ducts, or any other permanent use.)

There are, of course, other specialized tapes such as hockey tape, housewrap tape, etc.