I was playing with the CodeForces API and had a chance to cumulate the tag stats for various diff problems. Hope it helps you to prioritize the practice problem choice.

Diff: 1600 1 implementation: 109 2 greedy: 96 3 math: 84 4 brute-force: 51 5 constructive-algorithms: 50

Diff: 1700 1 implementation: 97 2 math: 91 3 greedy: 88 4 constructive-algorithms: 57 5 dp: 54

Diff: 1800 1 greedy: 96 2 implementation: 77 3 math: 67 4 dp: 63 5 constructive-algorithms: 51

Diff: 1900 1 dp: 91 2 greedy: 89 3 data-structures: 64 4 math: 60 5 implementation: 56

Diff: 2000 1 dp: 98 2 greedy: 86 3 math: 62 4 data-structures: 55 5 implementation: 52

Diff: 2100-2400 1 dp: 342 2 data-structures: 272 3 math: 229 4 greedy: 208 5 graphs: 196

Diff: 2500-2900 1 dp: 260 2 data-structures: 200 3 math: 166 4 graphs: 132 5 trees: 114

Diff: 3000-4000 1 data-structures: 119 2 dp: 101 3 math: 88 4 trees: 62 5 graphs: 61