Pursuit can be argued to have the highest priority in all games it appears in, as it has absolute priority when the target attempts to switch. However, conventional wisdom would have it that Pursuit bypasses priority altogether, such that its "priority" shouldn't be compared to that of other moves. If you subscribe to that line of thought, then Helping Hand has the highest priority in all Pokemon games it appears in, with +5.
In Gen 2, the highest priority is a three-way tie between Detect, Endure, and Protect, all at +2. In Gen 1, Quick Attack reigns supreme at +1.
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