Fortress isnt actually bad if you play it the way its meant to play - making your units get hit and retaliating. And the units you get there are top notch, some tiers are easily one of the if not the best in the game. Gorgons anyone? Chaos hydras?
Nah, Fortress is still one of the weaker factions in the game. I'll give you Mighty Gorgons for their ability to randomly kill off Tier 7s, but Hydras are slow as shit walkers and Sonic Sez that's no good.
Of course, it's kind of hard to "tier" factions in the game, because someone is going to show up and go "in situation X, faction Y is the best!" e.g What if Dungeon gets resource screwed, then the low maintenence factions will win! (which I find unsatisfying, it's like winning by Mana Screw in M:tG)
Which is good on the part of the developers, but I think I can do it with "this faction has a high/low probability of winning overall." I'm not going to go into much detail, I'm lazy.
Borkked: Conflux
This faction was a hackjob thrown together at the last minute because some people got extremely pissy and mailed death threats to the devs for the gall of wanting to include a sci-fi faction in HoMM. What a shock that it's broken in six million ways (Tier 6 Hydras that are magic immune wtf).
OP on larger maps: Necro
Farm nine billion skeletons and go to town. That's on large maps, but on smaller maps they're still really solid because upgraded Vamps, Dread Knights and Liches are all really good units..
Consistently Good while still being strong late game:
Castle
Rampart
Both factions can play "cheaply" while still having options for ramping up if you find resources. Castle basically has the best Tier 7 unit in the game and other solid units.
Rampart's kind of a weaker Castle IMO, barring niche circumstances.
Both factions have a ranged units that get Double Shot as an upgrade making creeping a breeze.
Strong but susceptible to resource screw:
Tower
Dungeon
See title.
Dungeon's really bad at this (the Beholder's building seems to be custom tailored to piss off people at Impossible difficulty with its "1 of every resource" cost). Cool special buildings though!
"Cheap" Factions:
Stronghold
Fortress
See title. Better stomp someone's face early and conquer a 2nd town ASAP to get stuff you need like more spell support!
I put Stronghold over Fortress because they can do things like rush Behemoths more easily compared to Fortress. Also they got a unit that can siege walls, which Fortress REALLY REALLY wants to do.
Fortress has some niche advantages like being good on Swamp Terrain, Tazar being a completely nuts hero with how he turns your units into walls, etc. Bad match-up against Necro sucks though! (Undead are immune to Mighty Gorgons special)
Red Headed Stepchild:
Inferno
Bad stats overall, wrong element focus (Fire's the worst), lacking specials (Yes, I totally want Magogs to complicate things!). Not resource-cheap.