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Hive Guard (Lurk): Always a solid choice. Having T6 and 2 Wounds each, they actually do basically count as 2 Missile Launchers for 50 points apiece. Lurk means they don't totally suck when they leave Synapse because you do actually want them shooting their ranged weapons as often as they can. Pretty much one of the better choices regardless of army build and points limit.
Lictor (Lurk): Ew. Chameleonic Skin would be okay, if Pheramone Trail applied to itself, but it doesn't. Which means you need Hive Commanders or Swarmlord to stack, and if you're going to be doing that, well, you've already got a Hive Commander or Swarmlord to do the Reserve manipulation already instead. S6 Rending Flesh Hooks would be okay, if it wasn't AP- and the Lictor wasn't BS3. Flesh Hooks are not a Demo Charge. Lictors are not Marbo. Given that Lictors will almost always be deployed outside Synapse, 'Lurking' is exactly what Lictors don't want to be doing. Unless you plan on investing heavily in Mycetic Spores, Lictors are otherwise terrible.
Deathleaper (Lurk): Coming at the cost of two and bit Lictors, this guy comes with all the downsides of normal Lictors (which is a lot), but doesn't have the extra wounds that two Lictors would have. Ultimately, you're paying a whole lot of points to reduce an enemy ICs Leadership by D3 (roll a '1'). It can be okay when facing Librarians, but it isn't so much an issue when you already have Shadow in the Warp. Basically just as terrible as a Lictor, except you're paying for two and half of them which you wouldn't normally do anyway.
Venomthropes (Feed): It all depends on how many units you plan to have, and how much natural cover is available for your shooty units and whether or not you plan to run anything expensive (i.e; Warriors) up the board. Warriors are one of the main units that will benefit from this guy, but, so will your Monstrous Creatures, who, without 2+ saves, nor Invulnerables generally need all the help they can get. Keep in mind that Venomthropes are a pain in the arse for your opponent and they probably will end up targeting it - a lot. To that end, it's often helpful to keep them out of LoS (their cloud still works), or to walk them behind Monstrous Creatures (Carnifii and Hive Tyrants love Venomthropes). If anything, a Venomthrope should always have a 4+ cover save by being behind another unit. Good unit. But only if you need it. Feeding shouldn't usually be a problem since the things that most need Cover Saves (who the Venomthropes should be babysitting) nearly all have Synapse.
Zoanthropes: Note the Synapse. The difference between these guys and Hive Guard is +10 points for less toughness, and needing LoS to shoot (and, in return, being in opposing LoS). Warp Lance is okay, Warp Blast isn't that great. The other problem being the problem that all psychic shooting attacks have; Roll test. Roll To Hit. Instead of just rolling To Hit straight up (Hive Guard). The other thing to note is that both Lance and Blast are within range of Psychic Hoods which generally screw over every single Zoanthrope ever, and Runes of Warding are even worse. They can be decent in a Mycetic Spore, but, Zoanthropes are surprisingly fragile given their 3+ Invulnerable, and using a Mycetic Spore usually just drops them in the middle of your opponent's formation. So I hope your incredibly difficult powers go off...Even though you could pay less points for more durability and reliability...
Zoanthropes hit hard and die easily. Hive Guard hit reasonably and don't die easily for less points. Nearly everyone prefers Hive Guard because in the current meta Psychic Hoods are basically standard. And Hive Guard ignore Cover which is basically awesome on toast.
Doom of Malan'tai (Feed): Half again the cost of a Zoanthrope. Spirit Leech and Absorb Life are fine. Except they aren't. Spirit Leech allows cover saves, and doesn't work at all against anything in a vehicle (even Open-Topped ones), which, sadly, is the majority of the meta-game. Malan'tai must be in a Mycetic Spore, otherwise your opponent will piss themselves and aim everything at Malan'tai, that'll happen anyway once you land, but it's better than losing him on the first turn and not doing anything. Most of the time, Spirit Leech will happen once, so it had better count. Cataclysm is decent, but, remember that you're using a Mycetic Spore (right?) and Malan'tai is not Synapse (if I had 5c everytime someone assumed that he did...I'd have like, a dollar), which means if he fails his check (Ld10, thankfully) he can't use Cataclysm because he'll be affected by totally-stupid-Tyranid-version of Rage where he can't shoot.
Glass Cannon. And Glass Cannons are not good.
Pyrovores (Feed): Just bad. None of the Tyranid players I speak to have ever found a use for them (3 in a Mycetic Spore seems to be the only choice, and it still isn't a good one). And this author has never found them particularly threatening. Feel free to totally ignore. Due to having halfway decent ranged weapons and being absolutely mediocre in Assault, Feed is generally annoying.
Ymgarl Genestealers: I'm yet to hear anyone say that these guys are anywhere even close to being not useful. Again - like Lictors - these guys have the problem of being mandatory Reserves, meaning that you really need to take Hivemander or Swarmlord for them to be better than they otherwise are (waiting 'til Turn 5 for Reserves is just terrible), and that's not necessarily a bad thing, mind. Just something to think about. When they come out, it's probably best to use the Toughness on the turn they come out, that way they win less (they should never, ever, ever lose combat) and don't completely wipe out the unit they Assault just to be shot at in your opponent's next turn. S5 Rending is also okay against Vehicles and unlike regular Genestealers, their 4+ save means they aren't insta-gibbed by ubiquitous Bolters. Despite being 'named', Ymgarls aren't even Unique.
Ultimately, Zoanthropes and Hive Guard fill the same role. You don't need both. Hive Guard are cheaper and live longer if I haven't made that absolutely clear yet. Venomthropes are for when you play with particularly expensive Monstrous Creatures (like groups of Carnifii) or bricks of Warriors that you absolutely want to get in combat / not die, Venomthropes are also 'better' when you play in a meta with less terrain than you probably should have. Ymgarls will always have a place in any army - if you want them.