It depends on what you're trying to build.
You should never have more than six ripper swarms, as you'd only want to use them for their low points cost. Hormagaunts are generally underpowered for their points cost. They get into CC, tie up units, and die slowly. Spinegaunts do the same thing (less the 12 inch assault) for half the price. If you're into swarming with small gaunts, spinegaunts are the best.
If you want to swarm with genestealers, you're going to need a couple synapse creatures (I'd lean more toward warriors or a shooting hive tyrant than the broodlord, since he prevents his squad from fleeting and costs a lot of points with his retinue). Genestealer armies need to have enough guns to deal with skimmer vehicles, as well as be able to negotiate cover and difficult terrain effectively.
Lictors are generally terrible and should almost never be played. They only get four attacks on the charge, and are essentially a marginally better genestealer with two wounds. By turn two when you deepstrike in (and certainly by turn 3), you should be in CC. The only situation in which you might want to use lictors is in games with strange board edges or abnormally large boards.
Then you come to big bugs. This army is very very very expensive to build (if I retailed the carnifexes, they'd be 240, plus the 2 tyrants and one squad of guard would be about 360 bucks, then 15 raveners would get you near 600 for an 1850 list). That said, no one in their right mind pays retail. Anyway, I've found that raveners to be much better in the big bug army than genestealers, since they can be screened by carnifexes on the first turn, then come out and assault anything if it moved toward you on turn two. For shooting purposes, the ravener brood is going to be easier to hide than 12 genestealers, and is also divided by two wounds per model. The drawbacks are the -1 initiative (which can really be a problem against a couple specific lists, but generally doesn't matter), not nearly as many attacks (if we assume that somehow your genestealers don't take heavy casualties, and all have scything talons), and increased vulnerability to heavy weapons fire (generally the bigger threat is strength 5 and 6 stuff anyway, and that slays genestealers equally as bad). Raveners give you a new wrinkle in the list, sacrificing a little raw power for a lot more flexibility.
Big bugs are, in my experience, the best of the lists. It is nearly impossible to take away the threat of the big bugs other than to kill their mobility in six turns. Swarm armies are going to need genestealers to mop-up once the gaunts start dying. Mobile armies are a huge problem for genestealer armies.
