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