Most effective hero?

I have heard that the Warrior is the easiest by far for new players due to easily blasting through lower levels; however, after level 15 the Warrior's performance is significantly hindered and eventually drained once all of the health potions are consumed.

The Rogue, on the other hand, can be very useful in the higher levels, as can the Mage/Huntress. What I'm wondering is which hero is most effective for beating the game (reaching level 26).

I think it's more dependant on what items you have than class after a few playthroughs - before getting a new phone I had unlocked the huntress (by using a Rogue) and gained books on the three characters except the huntress.

I personally found the huntress most difficult to play, Warrior and Rogue were equally easy - Mage wasn't too bad either. Warrior is just an easy start due to the extra 1 Stamina.

I found that if I had a high armour of entanglement and 3-4 different battle wands I could easily get past the first two bosses on any of the three original characters.

Countering your experience my Warrior had the highest level achieved - using wands, having plenty of food (teleportation wand + piranha is an easy way to get food), and either unenchanted armour OR entanglement armour (found most other armours to hinder my gameplay but that is not the case for everyone).

I've not experienced an issue with lack of Health Pots, but when you pass the second boss or so I found my Mage, Rogue and Warrior to be rather similar in style as I would have collected wands on all of them to aid gameplay.

I wouldn't put any character above others (exception huntress but that is likely my inexperience with the character) as towards end of game you can have them built almost identically.

I'd argue that the Warlock branch of the Mage is the easiest.

Play it exactly the same as the warrior. It's slightly more difficult to start due to the strength disadvantage but the health and hunger gained every time you make a kill quickly overcome it.

Like all characters you'll need to be lucky and find some decent armour (often from the ghost) ol a good weapon before fighting the goo.

[I've finished the game with a Warlock, made it to level 25 with a Paladin and have the books for all classes.]

Maybe it's just the way I'm playing, or the variant I'm playing (Shattered) but they all seem to converge on being a good warrior in the end, or at least by level 15, in the sense of having a good weapon and armor. The warrior is probably the most effective, but once you get past level 10 or so the difference becomes minor. Again for Shattered Pixel Dungeon only, so other variants might differ.

The Huntress is perhaps the most fun because it requires more technique. Throw inert objects (e.g., rations) into doorways to scout ahead and hit enemies multiple times. The easiest path seems to be using the boomerang for the first levels but then start to lean on regular weaponry later on. There's a food bonus for the huntress since you can kill all piranhas with the boomerang for meat. Infusions on the boomerang can be pretty cool - got a stunning boomerang once, which made it quite difficult for anyone to get near me.

The Warrior is the most straightforward. Keep all your upgrades for the first non-cursed scale or plate you find. If you can get like +4 scale by, say, level 7, then you start saving most of your healing potions for getting through levels 15+. It also might work to upgrade a tier 4/5 weapon first; I had a fun game where I found a +2 glaive early on, poured most of my upgrade scrolls into it, and just killed most enemies with one or two hits. Armor is less important when no one gets an opportunity to attack you. Either way, the added start strength means you can use more fun stuff sooner.

The Rogue is fun for the door hiding as an assassin, as others say, and also cloaking to get guaranteed hits. That's not as much of an advantage at the start of the game as the wand or the boomerang, so starting out with this guy sometimes seems like a bit more of a struggle.

The Mage has the staff, but it doesn't scale well -- not as much fun at +5 as a +5 warhammer is. It's always a tier-1 weapon, so its damage does not scale as fast. Helpful at the beginning, not so much later. I never understood the point of infusing it with another wand -- I'd rather have two wands than one. Warlock is a nice way to level way up since it gets you lots of free food and healing. Especially fun if you can mark and then use a wand of corruption.