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