I would say hands down BTC and then .0XX01 denominations. Newcomers will find mBTC and uBTC a little confusing and sometime I forgot what they stand for. This isn't even considering the more uncommon denominations. However, what I do think would be nice to make things more clear are more integrated BTC to USD/EURO/RUS converters. That way people have an easier way of visualizing what .0158 bitcoins is actually worth.