These are the list of heroes I find particularly useful when dealing with difficult PVE situations.
1. Human Torch
Ever since the update that gave him Plasma Body and Fire On!, Human Torch has become a one-man wrecking crew capable of taking out entire batches of opponents by himself. He even heals himself when resting, so when high scores aren't a priority, he is awfully useful. His immunity to fire is especially handy when fighting Dormmammu and others, and high attack allows him to one-shot most bruiser mobs. Against Dormmammu, his 1st lvl attack gets rid of the dancing flames and sets up fire making it very easy for anyone to deal with him.
The downside is that ALL of his attacks are Ranged Fire Energy, so there are instances when he just can't do anything (fire immunity). But other than that, his attack can be pumped up to such a level that even tacticians can't really take advantage of his low defense (which is also mitigated by his blazing speed that increases evasion for ranged attack and plasma body which deals damage to melee attackers).
2. Invisible Woman
As a tag-team with Human Torch, she supports what the Torch lacks. Defense is supplemented by her shield, and tacticians are taken care of as she is an infiltrator. She also gets a PVE bonus with Torch (Fantastic and Sibling Rivalry) making it quite desirable to pair her with the Torch. Her attacks are also all ranged, but at least they're not energy and they don't trigger counter attacks, so it also supplements something that the Torch lacks. As she is a mandatory teamup against Vapor (4.4 farming) and the Torch can take out the bruiser in one shot, this combination is worth pursuing. Her Force Cage is also very neat when dealing with side missions that require certain skills to be used a certain number of times.
I honestly don't see much of a disadvantage in using her as her attacks are fairly strong as well, and her force cage can be a life saver in tricky situations.
Normally, those two usually can handle everything there is to PVE, but there is one other worth mentioning.
3. Scarlett Witch
Invisible Woman's force cage does not prevent debuffs from being applied, so if the opponent is really tricky, it is possible to wear him down using her probability field and IW's force cage. It gets really tedious,
4. Scrappers/Infiltrators for 4.4
As Vapor is an Infiltrator, and Magneto a Tactician, a Scrapper against Vapor and Infiltrator against Magneto really speeds up the process. Also they give 140 and 200 xp each, so infiltrators and scrappers can easily level off boss fights as well. Vapor especially is very easily handled with a scrapper, most notably Ghost Rider who can apply a ton of debuffs that Vapor cannot handle - and he can always switch from melee to energy depending on Vapor's status. The Scrapper's second attack will not trigger a counter-attack from Vapor.
Magneto is more easily dealt with heroes that apply debuffs more than the class of the heroes, but infiltrators do speed up the process as they'll retaliate every single time.