Oh, there's a morbid delight in watching somebody field an entire army that's made up of, like, five models that go straight into curb stomp mode, for sure! Really, a lot of the various factions lend themselves to throwing a whole mess of troops if you so desire: Eldar, Sisters of Battle, even Space Marines if you keep the Titans and Rhinos to a minimum.
Or at least this was the case back when I was more up-to-date on my edition lore. 4th, I think, was the last rule set I was familiar with, and I think they're up past 6th at this point, so take anything I have to say with a grain of salt. :)
8th ed. now. Sorta-kinda the same, but new hyper-elite armies have been added to the mix.
Guard blob is still top-tier. Nids and Orks are more variable. Soace Marines of various flavors are still solid, and the new Primaris marines get all the love nowadays, but the Classic Marines can pull their own weight just fine. Chaos has improved. Sisters of Battle are getting a re-release just now. Necrons are a bit stagnant, but they look cooler than ever. Tau still excell at shooting and become very squishy if you manage to close in.
New stuff:
Adeptus Mechanicus is really cool. Cyborg guard with radioactive guns and robotic support.
Imperial Knights are big stompy mechs with big guns. 4-5 models can make a 2000 point army. They can also ally with pretty much any Imperial faction in a combined arms strategy.
Custodian Guard are to Space Marines what Space Marines are to guardsmen. They uave a lot of armor, several wounds apiece, and they dish out the punishment like no one else. But they can be swarmed, and can have trouble controlling the board and capturing objectives. And they can take Sisters of Silence as allies to add numbers and deal with enemy psykers.
Genestealer Cults blend Imperial Guard nymbers and Tyrannid biology in scary ways.