A 1.25 oz pump spray of 100% DEET.

This is the maximum-strength option in a size that goes everywhere. Mosquitoes at dusk by water, black flies, or anywhere the bugs are bad enough that ordinary repellent stops being funny.

Higher concentration does not repel harder, it repels longer. 30% covers an evening comfortably; 100% is for all-day exposure or genuinely bad conditions. At 100%, protection runs most of a day from one application.

Apply to exposed skin only and sparingly — more is not better.

One warning worth knowing: DEET attacks plastics and synthetics. It will haze a watch crystal, cloud sunglasses, soften a rain jacket's coating and weaken fishing line. Spray it on your hands away from your gear, and wash your hands before you handle anything you care about. At 100% concentration this matters more, not less.