2 and 3 are indeed good, with 3 being my favourite, but I can also suggest 5. On its own, it's 3 with a facelift and some touch ups , so it's good. However, if you get the Tribes of the East expansion, it becomes awesome - it's very slightly behind HoMM 3 in my list, and not gameplay-wise[footnote]frankly, it's just that you can play HoMM 3 on a toaster, so it's easy to just play it on a laptop. Also, random map generation is faster. Simply means it's easier and faster to start a game, which is also good to play with friends[/footnote]. As for the rest - 4 is different. Not necessarily bad but...well, it's really more of a different game. You can still play it, I guess and try it out. And finally, we have 6. I am pleased with 6 but I am not sure where I'd class it. It's a more of a strategy game, is what I can say.
I'd say get the games from GOG. If you for some reason want one, I'd go with 5, since you're a new player, otherwise it'd still be my top recommendation. For a weird reason it's both a good entry into the series and not such a good one - on one hand, you at least get explanations what abilities do and stuff - having tooltips is really a good thing. On the other hand, it improves upon the older games and it even takes the good parts of 4, so it's slightly redundant to play older games at that point. But if you can get a Heroes bundle definitely do that. The good thing about GOG is that if you get, say Heroes 2 and 3 and suddenly the Heroes bundle is on sale, you can simply get the other 3 games for the same reduced price - you don't need to re-buy the ones you have. At any rate, I'd go with 5, probably, and you might also want to try 4 - you might like it (or not, dunno) - as I said, it's more of a different game, really. If you're not bothered about playing older games, get 2 and/or 3 as well and you can even start with them. At any rate, once you get some playtime under your belt, and form an opinion what you like and what not, you can try the Heroes 6 demo. At that point you will decide if it's for you or should you skip it.