In my opionion there is no best shield. It depends on your playstyle and build.
If you want to play very offensive try one of these:
- Knightly Heater/Kite Shield: has nice speed and armor, but the protected area is too small for me, often got hit by arrows or in melee even if I had the shield up, breaks very easily against shield breakers (you can put the shield on your back in 1on1 but against several enemies thats not an option)
- Elite Cavalry Shield: nice speed and good protection, but can't take lots of hits, perhaps still my prefered light shield, very nice for cavalry
For a more defensive playstyle I like:
- Heavy Round Shield: 1 slot and long duration even against shield breakers and awesome protection, if you want to use a sidearm (like a spear) I would use this shield, otherwise I would go for the Huscarl's
- Huscarl's Round Shield: like the Heavy Round with even better stats, but 2 slots, was very impressed when I tryed it out, perhaps my prefered shield
- Plate Covered Round Shield: very long duration against non shield breaking weapons, but rather slow and low hp
Allround Shield:
- Brown Lion Heater Shield: high hp, ok armor and nice speed for a very good price, but like Knightly Heater/Kite a rahter small protection area. You can't do something wrong with this shield, but the look is terrible.
You should always also look at the armor you wear, your athletics and your shield weight together with your weapon length. Don't make yourself too slow.
What I really don't understand, is why people prefer the Round Shield. The Heavy Round Shield is nearly the same, only much better.