Sturmgewehr

I would break it down like this.

Modern Era:
1) M16
2) AK-47 / AKM
3) FN FAL
4) G3/HK 91
5) G36
6) Steyr AUG
7) Sig 550
8) Galil
9) FAMAS
10) M14

I put the M14 last because while a neat rifle, it was antiquated the day the first one rolled off the assembly line. It was eclipsed by the FAL, which we should have adopted.

Best modern (WWI - present) rifles:

1) M1 Garand
2) Mauser 98/k
3) StG 44 / MP 44
4) M16
5) AK47 / AKM

I'll stop there as the list will get repetitive. :)