First of all, aliens need to make some _investment_ to be able to hurt bases at all. Humans don't really need, they can attack with machineguns if they feel like it. Humans have much easier time attacking a base. And don't forget about grenades. Grenades can really slaughter bases, you don't even need the line of sight. Other than bases, they're not very good (cost-effective) unless used in bottlenecks against entire alien swarms. Throw it at attacking dragoon, there's a good chance he'll be caught in explosion on his way back, when he's too excited about killing someone to use caution while retreating.
My experience is that aliens totally own humans with tyrants UNLESS humans abuse certain base spots. There are places which easily lead to a draw (basically inaccessible by tyrants). These places are... ummm.... how do you call this map.... the one with some snow outside, and small elevator room. Elevator room human base can sucessfully defend against tyrants and lead to total stalemate/draw.
Another recently discovered base spot, on another map (I have NO IDEA why no one tried it before) is the one above big room with crates. The platform above crates. You find this place by going through upper human base entrance (initial). It's the map with lots of pillars near initial alien base. Once I actually got my team to try the spot, we laughed at tyrants, and alien team was clearly pissed off. Multiple coordinated dragoons and marauders had very hard time jumping up there. I'm confused, because this place clearly looks like made for building a base, and I haven't seen anyone build there before.
Other than these 2 spots/maps, humans seem to HAVE TO rush to win. Each time I see tyrants, humans fall sooner or later. Except perhaps Uncreation, but that map is totally broken in favor of humans (with big teams). Jetpacks and lucifer spam....
Even on transit. It might look like humans would have easy time there, because it's full of empty spaces and you can really abuse jetpacks. But marauders are almost unkillable there, and tyrants eat what's left.
I have to say aliens are much harder to adapt initially. With dretches, you should use wallwalking as much as possible, otherwise you're too easy to hit.
As basilisk, you should evolve to another lifeform, period. Prove me wrong. Not as fast as dretch or marauder, not as tough as dragoon, marauder or tyrant, and attacks are unimpressive. Very ineffective against battlesuits AND jetpacks. Something should be done about basilisks, they're especially ineffective with bigger teams/games.
My suggestion: either make basilisks tougher (probably not a good idea), or make them better opportunists.
Making them better opportunists would include:
Enable basilisks to see health bars of enemies. This would make it easy to judge if there's any point in attacking a battlesuit or just enemy in general.
Make basilisk's 'hold' ability work as well on battlesuits as on regular marines. They really don't deserve to be so crappy against battlesuit, one attack is totally useless against them, and basilisk's melee is already weak enough. All other aliens are easier to play.
Make basilisks more silent, especially jumping sound.
Perhaps give them short-ranged wallhack ability of some sort.
Basilisks really need a boost, you know.
As for my favourite alien, it would be marauder. Fairly resistant, very quick (as long as you jump around like an idiot, you're almost unkillable, even if you can't hit anything). Chain lightning is nice against crowds, flamers, painsaw, and clusters of buildings. Marauder isn't very good in vertical jumping/travel, but everything comes at a price. Their horizontal speed more than makes up for that. Marauder is probably the easiest alien for a newbie.
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One last thing: alien defensive structures are unimpressive. Acid tube is good. Trapper is very situational, and requires some knowledge to use. But I have to say it can be suprisingly effective, for example when combined with acid tube in ATCS middle of map building. Hives are just disappointing, attack only one enemy at a time, deal unimpressive damage, and, what's unforgivable - COST MORE than acid tube. They should cloud victim's vision or something.
Oh, yes, and the barricades. I almost forgot about them, which probably proves how pointless and underused they are.
