Some observations about the top 100 German words:
• The top 10 words make up 27% of words that occur in German. The top 20 words account for about 35% of words that appear in German written and spoken texts.

• The top 100 German words contain only 4 nouns, three of which have to do with time.

• The top 100 German words include 30 different adverbs, more than 20 pronouns, and 16 prepositions. Small function words thus play a dominant role in making sense of German texts.

• The top 100 German words list 16 different verbs. These include 4 of the 6 modal verbs. Also included are the verbs commonly used as auxiliary verbs in German, suggesting that compound verb tenses are used relatively frequently in German.

The word list below is based on a scientific study conducted by Randall Jones & Erwin Tschirner. For a much longer list of the most common 4034 German words (!), consult their book A Frequency Dictionary of German. Core Vocabulary for Learners.

1. der / die / das (def. art.) the; (dem. pron.) that, those; (rel. pron) who, that
2. und (conj.) and
3. sein (verb) to be; (aux./perfect tense)
4. in (prep.) in [variation: im in the]
5. ein (indef. art.) a, an; (pron.) one (of)
6. zu (prep.) to, at; (adv.) too
7. haben (verb) to have; (aux./perfect tense)
8. ich (pers. pron.) I
9. werden (verb) to become; (aux./future tense); ; (aux./passive voice)
10. sie (pron.) she, her; they, them; Sie (pron.) you (formal)
11. von (prep.) from, of
12. nicht (adv.) not
13. mit (prep.) with
14. es (pron.) it
15. sich (refl. pron.) -self
16. auch (adv.) also, too
17. auf (prep.) on, at, in
18. für (prep.) for
19. an (prep.) at, on [variation: am at/on the]
20. er (pron.) he
21. so (adv.) so; thus, this way, such
22. dass (conj.) that
23. können (verb) can, to be able
24. dies (pron.) this, that
25. als (conj.) as, when; (adv.) than
26. ihr (pron.) you, her; (poss. adj.) her, their, hers, theirs; Ihr (poss. adj.) your (formal)
27. ja (adv.) yes; certainly, really
28. wie (adv.) how; as
29. bei (prep.) by, with, at
30. oder (conj.) or
31. wir (pron.) we
32. aber (conj.) but; (adv./flavoring particle)
33. dann (adv.) then
34. man (pron.) one, you
35. da (adv.) there; (conj.) because
36. sein (poss. adj.) his, its
37. noch (adv.) still, yet
38. nach (prep.) after, toward
39. was (pron.) what
40. also (adv.) so, therefore
41. aus (prep.) out, out of, from
42. all (pron.) all
43. wenn (conj.) if, when
44. nur (adv.) only
45. müssen (verb) to have to, must
46. sagen (verb) to say
47. um (prep.) around, at [variation: um ... zu in order to]
48. über (prep.) above, over, about
49. machen (verb) to do, make
50. kein (pron.) no, not a/an
51. Jahr das, -e (noun) year
52. du (pron.) you
53. mein (poss. adj.) my
54. schon (adv.) already; (adv./flavoring particle)
55. vor (prep.) in front of, before, ago
56. durch (prep.) through
57. geben (verb) to give
58. mehr (adv.) more
59. andere, anderer, anderes (adj./pron.) other
60. viel (adj./pron.) much, a lot, many
61. kommen (verb) to come
62. jetzt (adv.) now
63. sollen (verb) should, ought to
64. mir (pron.) me
65. wollen (verb) to want
66. ganz (adj.) whole, all the; (adv.) quite
67. mich (pron.) me
68. immer (adv.) always
69. gehen (verb) to go
70. sehr (adv.) very
71. hier (adv) here
72. doch (adv.) however, still; (adv./flavoring particle)
73. bis (prep./conj.) until
74. groß (adj.) big, large, great
75. wieder (adv.) again
76. Mal das, -e (noun) time; mal (conj.) times [math]; (adv.) time(s); once; just
77. zwei (num.) two
78. gut (adj.) good
79. wissen (verb) to know
80. neu (adj.) new
81. sehen (verb) to see
82. lassen (verb) to let, allow, have (something) done
83. uns (pron.) us
84. weil (conj.) because
85. unter (prep.) under
86. denn (conj.) because; (adv./flavoring particle)
87. stehen (verb) to stand
88. jede, jeder, jedes (art./pron.) every, each
89. Beispiel, das, -e (noun) example
90. Zeit, die (noun) time
91. erste, erster, erstes (adj.) first
92. ihm (pron.) him, it
93. ihn (pron.) him
94. wo (adv.) where
95. lang (adj.) long; lange (adv.) for a long time
96. eigentlich (adv.) actually; (adj.) actual, real
97. damit (adv.) with it; (conj.) so that
98. selbst, selber (pron.) -self; (adv.) even
99. unser (poss. adj.) our
100. oben (adv.) above, up there

Source:

A Frequency Dictionary of German. Core Vocabulary for Learners, by Randall Jones & Erwin Tschirner, 189 p. (2006)
This provides a frequency list of the most common 4034 words in the German language based on a broad text corpus of contemporary German. Each entry lists the word's rank, part of speech, its English translation, a sentence demonstrating the word's usage in context, and the number of occurrences of the word per million words of the corpus. Entries also list common collocations, phrases, or combinations. In addition, the dictionary contains much supplemental information that students and instructors will find useful. There is an additional alphabetical list of the 4034 words, ordered lists of the top 100 most frequent German nouns, verbs, adjectives, adverbs, proper nouns, and irregular verbs, and additional lists of the most common prepositions, conjunctions, abbreviations, and collocations. Of interest are also the 21 thematically organized lists interspersed throughout the book (e.g., numbers, colors, weather, personal pronouns, kinship terms) that rank and provide frequencies of the included words. This is a useful book for instructors and students alike.