Although i usually use a pure ranger for my first hero (for fast exploration & diplomacy), i really love the commander class. My favorites are commander->ranger and commander->wizard:

~ The Commander->Ranger ~

lvl 10: add +2 to ranged attack for all
lvl 20: add another +2 to ranged attack for all
-> with the lvl 3 ranged skill from commander, you can add +6 to ranged attack for all your ranged units. However, the game uses only the base value to calculate the necessary damage for sniper medals. I.e., your elven archers with dual shoot deal 15+ damage for each attack (and some precise shot boni, fire arrows later). They also don't die all the time due to the defensive commander options. The Ranger-Commander is actually better than a pure ranger in single shot mode (+6 ranged damage vs. +4 from lvl4/lvl5 skill), and using a crossbow he won't miss the last two levels of precise shot much.

~ The Commander->Mage ~

The ability to cast long-lasting, high level buff/debuff spells in addition to the normal lvl 3 commander buffs will create an army even more formidable than that of a pure commander. Battlefield control spells will make impossible fitghts easy by fighting half the army later etc. And you can skip useless skills like necromancy & summoning (and probably wand mastery, i have yet to see a fight last more turns than the number of spell slots).

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For my third hero, i will usually take warrior->mage, which allows (due to long lasting buffs) for a better solo fighter than a pure warrior as well as access to battlefield control spells, stamina spells etc.

I also forgot to mention the Scout/Adventurer/Mercenary's ability to swap weapons with no action cost. Very useful, especially if you forget to swap him back to his bow after a battle, which is something I occasionally do.

The only way it could be better is if he could swap weapons after using up all his action points, but that would make him too powerful, I suppose. Attack with huge sword, then switch instantly to first strike weapon for defense, switch back to huge sword, attack, rinse and repeat. Although you could limit it by allowing only one weapon swap per turn...hmm.

I tried warrior -> Mage having the same thoughts abouts buffs. But it sucks. Even with the buffs its not better than a berserker, because you lack lots of stats from the skills. Then enemies cast dispell on your buffs. I even lost very clearly to the enemy berserker, even though I have some good equipment and incarnation!

This is why i take warrior->mage as a third hero only. He is cheap to maintain, levels up fast (solo), and can easily earn additional money & items in the backyard to maintain my main heroes & armies. I didn't use him much vs. enemy heroes so far, maybe cast some mass debuff first so the enemy tries to use dispel on his army first, idk how well that works. Anyway, vs. monsters without heroes, use mass slow, wait a few turns, press F10...

My prefered, mainly for roleplay as I used to play only "heavy scouts" (I would love playing it as a Wood Elf).
I take it as 1st hero, but it would be more usefull as second.
Swaping weapon is mainly a gadget: I almost never use other weapon than my good bow.

I am not sure here if we are talking about pure solo Berserk(can't remember lvl 20 name) or Dark/Death Knight here.

The main reason I have started to see the benefits of DK over Berserker is because the DK can get Magic 7 which gives him one T4 slot and two T3 slots which if he goes pure he cannot touch. Not to mention he can take 1 more T1/2 spells or even more if he gets Wisdom Mage skill. This means he can get and cast spells like Mass Slow (T3), Cloud of Terror (T4), Invulnerability (T4 would make Dragon fights easier I think) or even Reincarnation (T4) which is also the Paladins skill. Also any buff skill can get upto an extra +3 duration with Concentration.

I think the main things he loses out on not going "Pure" is an extra +3 Melee and Ranged Defense and +4 Melee Attack/Counter Atk. And most importantly Round attack with like 2 more parry and Armor Piercing. Charge and 1 more Speed are also lost.

If you actually care about carrying troops with you ofc the DK loses out major here as they only get 4 T1 and 2 T2 slots max for troops.

I was talking about the Death knight, and compared it to a berserker (pure warrior).

Death Knight doesn't suck, you're just not used to playing with one. They're very different from Berserker/Chieftains.

I wouldn't say DK is better than pure Berserker/Chieftain though. The pure warrior is a one-man wrecking crew, no doubt about it.

If you have had exp using both can you plz tell me some reasons why you don't think that the DK can be as effective as a Chief?

The only real reason I can see is Round Attack and I actually rarely used it when I actually bothered with going straight Zerk. This could be because I usually play Tiny to Average size shards and never get my Heroes past lvl 25ish really. before it's almost impossible not to win.

Well, I didn't say that either. I don't think one is necessarily better than the other, except maybe that Chieftains are simpler and easier to understand. You see things and smash them. The first few times I tried DK I spent trying to figure out all their powers and how to use them. I even tried one as a wand caster once. He was really good at it, too, but it's not the most effective use of the DKs abilities.