What does a fashionable gentle-rat choose when it wants to kill man-things? The choice is yours, but there are some options that are more efficient than others.
Monks (damage per model, vs 4+ armour)
Base 0.56 0.51
30+ 0.80 0.84
+Charge 1.20 1.4
+ Filth or Rabid 1.60 1.9
Staves are almost always better, especially the more buffs are applied. Getting the charge is crucial. Blades of course also profit by the aforementioned, are however a more stable choice for when things go awry. Due to the re-roll they are consistent even with low model counts.
From a practical standpoint it has to be said that running Staves won't make you popular (the only friend you will ever need is the horned rat anyway!) and is quite a chore. Sorting a hundred dice and more into four piles is just not enjoyable gameplay for most people. As long as Blades are a justifiable option I will run them.
Clanrats (damage per model, vs 4+ armour)
Swords Spears 20 swords equivalent in spears
Base 0.13 0.083 31
30+ 0.33 0.22 26
+ Gnash Bones 0.44 0.33 27
+ Warbringer CA 0.61 0.45 27
The important part is the third column. Imagine you get 20 swords into combat, how many spears would it take to get the same amount of damage? At it's core this tells you, unsurprisingly, that you need a couple more spears. Therein lies the problem. Spears can afford to lose fewer models than Swords. Yes, you could potentially do more fighting in three ranks, but giving the rate Clanrats die you probably won't.
In addition spears suffer more from penalties to hit. Going from 4 to 5 is a heavier loss than going from 3 to 4. (On the plus side: blocks with spears look great. )
Why would you even care how much damage Clanrats do? Well, used as 20-rat screening unit you don't. If we assume a points increase for Plague Monks in the future, running two 40-rat blocks with a Clawlord can be a viable alternative in the sense that your anvils do some damage in return and you diversify your win conditions.
Stormfiends (per model equipped, vs 4+)
Launcher (+ spark) 3 6
Warpfire is just mortal wounds = 1/2 models in range.
Ratling (10 shots, + spark) 4.4 8.9
Grinderfist 2.2 4.4
Doomflayer (+1 to hit) 3.7 5.8
Shocking Gauntlets 3.3 6.3 (if fishing for 6s, re-rolling only fails: 5.3)
Both melee variants are pretty much equal, concerning their average damage as well as their reliability (or lack thereof). Despite not regularly seeing them, i believe that a unit of 3 with Warpfire brings a lot to the table. There are few matchups where a Flamethrower will be a complete waste and even then you can use the model as a sacrificial lamb in the way the mainstream ranged unit uses it's melee bro. In particular builds that are already strong against MSU/single target (e.g. double bell with chain warlock and Jezzails) can profit from having an anti-horde option.
The only thing that has a hard time justifying itself is the Grinderfist. Failing to arrive 1/3 of the time is critical. As well as the general problem with deepstriking SF this way: Warpfire is out of range, the "normal build" needs to be buffed in order to do work. Unless you run some form of original build that tries to mass deepstrike most of their army, I wouldn't advise building it.
Deathmaster (vs 4+ armour)
Fighting Claws with: Sword of Judgment 5.3
Rune Blade 3.1
Clan Eshin units follow the classic fantasy archetype of the assassin, with one exception: they aren't deadly. The Deathmaster follows this philosophy strictly. He has great utility and can really surprise your opponent both by appearing out of nowhere as well as not doing anything once he appeared.
Lucky procs of SoJ can of course be good, but sacrificing your artifact slot (you are Skaven, you probably only have one) for not even 4 bonus mortal wounds is just not an amazing option. Vigordust Injector on Fiends, Monks or Vermin can do this far more reliably.
Master Moulder (vs 4+ armour)
A question of aesthetics, typical for clan Moulder. Some 3" fighting shenanigans are always appreciated and can give you that extra bit of damage to tip the tides. It probably won't however. Using enough Moulder units to justify a support hero for them means that you need to put some extra effort in though.
Given the length of the book there are surprisingly few choices hidden away and with the exception of Stormfiends none of them fundamentally change anything about the units. I do hope this was helpful to all of you losing sleep over Clanrat equipment!