Depends on the style I'm fighting.
Pre 2.0 I was a big fan of a heavy, 3/4" plywood, coffin shield for front line, give no ground, long sword, style; and heavy, metal sled, round shield for flanking, light infantry, short sword style.
I'm still experimenting with 2.0, but so far love a phalanx, plank, barely mobile, wall for front line; at least as an NPC.
Could never really get into light shields. If I have to move a lot to block I'd really rather have a heavy spear.