Wizards gain rituals more easily than any other class; they start with three rituals for free, and learn two more for free at 5th, 11th, 15th, 21st, and 25th level.
Bards only gain two rituals for free at first level, but they can cast one bardic ritual at no cost every day. At higher levels, they can cast two or three free bardic rituals per day. So they pay more up front than wizards, but they can cast for free, which wizards cannot do.
Other classes with free ritual casting are all more limited in one way or the other (up front costs or casting costs) than wizards or bards. For completeness:
Artificers gain Brew Potion, Disenchant Magic Item, Enchant Magic Item, and Make Whole for free at first level. They can use Disenchant Magic Item for free as often as they like.
Psions gain two rituals at level 1; one must be either Sending or Tenser's Floating Disk. Once per day, they can cast whichever of those two they chose. This is less choice than the bard has, and they can never cast their ritual more than once per day.
Invokers are exactly like psions as far as rituals go, except that their second free ritual is Hand of Fate. They can cast Hand of Fate once per day, and never more than once per day. Druids are like invokers, but have Animal Messenger instead of Hand of Fate.
There are also various feats which reduce costs of ritual casting, but since any class can take them, I don't think they represent a difference between classes. I will note that there are a few feats which allow PCs to make magic items with levels higher than themselves, but again, these are not class specific. They are often background specific, though.
You can not have a Warhorse as beast companion.
Beast Mastery: You gain a beast companion, chosen from one of these categories: bear, boar, cat, lizard, raptor, serpent, spider, wolf or horse. (Martial Power page 38 + Dragon 392)
A Horse Beast Companion is not a Warhorse in any way concering rules. Your DM could houserule a Warhorse is a horse, but I would advise against it. It would be overpowered, as a Warhorse is quite more powerful than any of the allowed types. (HP, trample, defenses, attack, damage)
The horse as a Beast Companion is weaker than a Warhorse for balance reasons. Changing HP and Defenses is not enough, change damage too, for trample as well.
Trample and Charger could be added at the cost of one feat each if they are usable at-will, or both for one feat if usable once per encounter. Still, trample should do 1d8 damage.
I would give you a free Saddle of Strength, it takes up the slot of the much better Impenetrable Barding anyway.
You need the Beast Rider feat for it just like with any other Beast Companion. For this reason and because of the better feat support, I would still go with a refluffed Bear.
Mark of Handling gives feat bonus to mounts, and feat bonus to beast companions. Feat bonuses do not stack, so no, it is not +4 speed +2 AC.
Best Answer
That is a very hard question to answer, because it depends on tier and what you mean by the numbers. However, here are my leading candidates.
To summarize: clerics win for huge single-target healing. Warlords probably have the most stamina. Bards are right up there with warlords, albeit via temps rather than pure heals; and shamans are unmatched for party healing.
(And of course, there's nothing wrong with the ardent or the runepriest. Runepriests give really good buffs to their party, which is the other half of the leader's role; ardents have a superb range of at-wills and also grant temps rather easily. A well-played leader speeds up combat by buffing allies along with healing.)