In my opinnion, a single massive transport is of limited utility.
1) Most planets have a single defending troop, and at most a medium port, which a single troop transport can handle at no risk.
2) Many empires will sometimes have 1-2 planets with a bundle of troops. Varies by empire, but "bundle" can be 10 troops, or 50 troops. No single transport intended for taking planets guarded by single troops can take this out, but it can be taken out by a mass raid of these ships, at no risk.
3) Most empires will have 1-2 planets defended by 1 LSP and up to 4 defensive bases (a "worst case" example I encountered was, if memory serves, 2 of these plus an MSP orbiting 3 moons of 2 gas giants, all in close proximity. Needless to say, waiting for the planets to move improved the situation considerably). A time coordinated raid of a single LSP + 4 defensive bases by 10 troop transports using fast recharge shields, and moving at 40+ speed may see 1-2 losses where the bases coordinate by luck on a single ship, but for the most part they will have too many targets to effectively take any real numbers down. While often these planets are defended by a 'bundle' of troops, they are also great places for enemy troop ships to stock up, so it's also possible to see a lone troop defending the planet -- once 1-2 troop ships deploy troops (time coordinated though, so everyone should hyper in at the same time and you should see about 3-6 deploy before the ownership change takes place), the bases are yours and the merchants are REALLY running.... :)
I've learned early targets for troop transports are planets with bases -- no bases for the enemy means no new construction (yes, it was a sucky place for me to be in. Having tried playing it, I mean to have my opponents in it as often as possible).