The question is way too hard and my answer would be different every time you asked me, so I've extended it a bit to make a Top 25. I've liked or loved all of the below enough over recent(ish) times for each of them to be considered a contenders to be in an all time top five. I know its a big list but I've never been very decisive, I'm sorry.
25. Art Brut
24. Kosheen
23. Hefner
22. The Knife
21. The X Ray Spex
20. Laura Marling
19. The Modern Lover
18. The Libertines
17. Ladyfuzz
16. The Kills
15. Kenickie
14. Sigur Ros
13. Blondie
12. Larrikin Love
11. Half Man Half Biscuit
10. Belle & Sebastian
09. Joy Division
08. Pulp
07. The White Stripes
06. The Specials
05. Yeah Yeah Yeahs
04. MIA
03. The Clash
02. The Smiths
01. Helen Love