That's a bit generalist. The Basilisk and Griffon are open-topped because the guns are a bit big and anyway if your opponent's shooting them, you've done something wrong (yes of course within 40k lots of people do shoot basilisks but that's because the battle lines are fairly flat, your basilisks really should be 200" or so behind the main line), and obviously orks and dark eldar have open-topped transports, enough said. Ravagers are really just transpost like more guns, what else? Land speeders, sentinels and salamanders, they're reconnaisance so want the extra field of vision (ok, so technically land speeders aren't open topped but you know what I mean), and I guess that's about it. If there are any more, they probably are just crummily built.