I went onto Pixiv and searched for every starter in all their forms, to count up the number of fanart for each stater's evolutionary line. Here's the results.

1. Serperior line (11497)
2. Charizard line (11062)
3. Samurott line (7599)
4. Delphox line (7009)
5. Typhlosion line (6286)

6. Greninja line (5963)
7. Venusaur line (5731)
8. Blaziken line (4564)
9. Decidueye line (4246)
10. Empoleon line (4195)

11. Swampert line (3973)
12. Meganium line (3832)
13. Blastoise line (3740)
14. Emboar line (3668)
15. Sceptile line (3102)

16. Chesnaught line (2874)
17. Feraligatr line (2617)
18. Primarina line (2501)
19. Incineroar line (2395)
20. Infernape line (1593)
21. Torterra line (1040)