I noticed that IV was the last one made by NWC... should I leave V out of my purchase?

I already own all of the main series through X and have been working my way through them (up to MM2 currently)

First point: in case you weren't aware, then be advised that HoMM is a very different genre than MM.

To answer your question: there are fans who would say that any given one of them are the best, with perhaps HoMM1 being the sole exception (primarily due to quality-of-life improvements of later entries). I've enjoyed all 5.

HoMM4 plays differently from the others in a number of ways, and a lot of people didn't like the mold break that occurred. HoMM5 is very similar to HoMM3, though it has a more 3d map.

I'd say HoMM5 is fine; go ahead and get it too. Or wait for a sale and pick up all five of them in a bundle.

GOG's entire colection of the Heroes games are on sale right now for $23.04. I did see that they are a different genre - turn based strategy... with RPG elements? I'm not sure what other games I would compare them to but they seem to be very highly regarded.

Ah! It didn't even register with me that they are currently on sale, since I already have them.

In a very broad sense, you will use heroes to move around the adventure map, collecting things lying on the ground, claiming structures to provide resources or other benefits to you, and fighting opponents. The heroes can level up, becoming stronger and better able to support their side in various ways (the biggest way is by better supporting their creatures in combat). In each HoMM, a hero always travels with an army (and only heroes can move around the adventure map), and the army does the actual fighting and dying in combat; HoMM4 is the exception, and the different role of heroes (as units on the battlefield) is part of what turned people off of it.

It has similarities to Age of Wonders, Warlords (not the Battlecry spin off), Master of Magic, Civilization, and Master of Orion. Some of these games are more similar (AoW), and some are less similar (Civ).

Heroes Chronicles is basically a set of extra campaigns for Heroes 3. You don't need one to play the other, but the engine and mechanics in Chronicles are straight HoMM3.

HoMM 1 is only worth it for the campaign, since HoMM 2 improves everything.
Chronicles should be played first since it's aimed at total noobs, and holds very little challenge if you have played the other titles.
2, 3 and 5 are all good games. Haven't played 4, but of 1-5 it's the game that deviates most from the formula.

I played all HoMM execpt HoMM V, and they all are good (can't say about V as didn't play it). HoMM IV is a bit different from previous games, but I like it for being different, the only issue with HoMM IV is that enemy's AI suffers from dementia.

However HOMAM IV has various mods/patches and the one I looked up fixes the AI, so you can always use a fanmade patch to fix the AI.

I'd say the single issue with HoMM IV in one word is "Combat". The secondary skill. Extremely unbalanced.

AI is bad, but then again the AI isn't stellar in all HoMM games. All other issues are a matter of taste (some I like, some I dislike, to some I'm indifferent). Despite all this however, I am enjoying the game. Worth buying and playing.