I'd use longbowmen, but they're so expensive I can't justify building them if I can build knights. I find it odd that a 4/3/2 knight costs the same as a 4/1/1 longbowman. I think from this my favorite is the Swordsman.
Longbowmen are the same as Medieval Infantry, so they aren't very useful if you have iron (unless the defensive freeshot means a lot to you).
I say the best is the tank. Before you get tanks, the best possible unit has 6 attack, not counting UUs. Then you get tanks, with 16 attack. That is a huge jump!
In my opinion, cavalry are the greater gamebreaker. If you play your cards right (and get a little lucky perhaps), the best defender your cavs should be facing is the musketman, with a defense of 4. The best your tanks should be facing is the infantry, at 10. The tank has a slightly better attack/opponent defense ratio, but that is more than offset by the fact that cities are likely to be larger in the tank era and many cities may also have civil defense. The attack/defense ratios of the units are the same, and cavalry have that all-important extra move point. Also, cavs fare more favorably against riflemen than tanks against mech infantry (and perhaps even TOW infantry, if you consider the increased city size and civil defense).