Terrans are the best on islands. As long as you can build enough marines in the beginning of the game, no air attack can hurt you. Comstats are also useful for setting up drops. Terrans shouldn't worry about attacking, rather they should plan to expand and contain. Getting 4-6 dropships full of marines is enough to win a game early on. Of course if this fails, you may opt to try again. Against the Zerg you should be able to build wraiths faster than he has mutalisks, so try to hunt down overlords or at least push them back home. You should also make sure that they don't land on your main island with hydralisks or reavers, so tanks might be required.
Constructing enough barracks is the most important thing. Marines are the best at taking out air, and can be produced in mass to prevent any drops. I usually don't like to tech too fast on an island game, since I'd rather get another expansion and more marines. Don't get me wrong, I do build starports, but only for dropships. not for mass wraiths. A few wraiths for scouting is essential though. I usually play islands, using only land units, that have good anti-air weapons. This is only because it makes it easier to attack and defend, yet you lose the mobility air units.
Now if your first drop failed, you should have just started your expansion at the same time. You should float your 1st factory to your expansion. So you should then construct another 2 factories and an armory. This is because you plan to produce goliaths. You should also get the range upgrade because this is the only reason why goliaths are good. Start constructing another command center. You should plan to drop a couple of goliaths off on another island closer to your opponent. This is to get a clear path to his main base, and to protect your own expansion. Try producing a few wraiths to cover all of your bases. 3-4 will do, just to push any guardians away. If you feel safe then use them to cover other possible expansions.
Creating masses of goliaths is also a good idea, since they'll easily wipe out battle cruisers and carriers. You should bring tanks when using them to drop on there main base. It becomes hard later in the game to use drops, since turrets, scourges, spore colonies, and cannons tend to become too dangerous.
You should start constructing more starports and a science facility. Start producing battle cruisers, and get ready to transfer you scvs to your new expansion. Prefferably build more factories if you can support them. You should be able to construct a fleet of 12 battle cruisers pretty fast. You may want to build science vessels and get irradiat instead, if it seems that you'll need to kill some units off before you attack. Also another option would be to uses nuclears across the water. This is especially useful since they'll have a hard time finding the dot. Overall you should use goliaths and missile turrets to take over the rest of the map, making sure they can't expand easily.
