Well, we have some nasty units in our arsenal;
Dakkatyrants = cheap and effective
Deathspitting warriors= 2 variants here, tl ds and +s, ds and scytals. I prefer tl deathspitters but both are deadly.
Fexes= Either for cc or ranged support. They are nasty, deadly, durable. Shooty ones tend to pay back their points more. CC ones solve problems which shooting cant, but have some issues as speed and low init, without any inv. save. Psychological factor is huge.
Gargoyles = Probably everything you can imagine on a "gaunt" base. Wings, fleshborers, bio-plasma, increased leadership with deepstrike ability. Costly in all ways, though.
Genestealers = rending nerfed, but feeder tendrils and scuttlers began something else, giving us a unit which has many tactical options.
In my opinion, there are two ways to make a list "top tier" Either by trying to create a perfect coherency and balance, using above units or by focusing on one aspect, magnifying it to an extend where it can ruin most balanced enemy armies, and while maintaining the basic "must"s such as scoring.
However after some point these lists became too popular. So some romantics tried to find the third way of reaching bliss, by findin some neat combinations, original or crazy ideas. These tend to shine and fade, and such solutions are not bright versus some situations.
say, a warrior and gaunt heavy army as I discuss in another topic, while having huge power against ork-like hoard armies, It has weaknesses against vehicle heavy armies.
There are quite-a many ways to devour enemy biomass and assimilate it into pure beauty and perfectness, I guess.