I'm playing a (currently 5th-level) human beastmaster ranger with a boar companion.
Str 18
Con 14
Dex 14
Int 10
Wis 14
Cha 10
Trained Skills: Acrobatics, Athletics, Endurance, Nature, Perception, Stealth (human bonus skill), Thievery (Sneak of Shadows bonus skill)
Feats (4 total, including human bonus feat):
Durable - for the extra surges, since the beast eats up healing like crazy
Wpn Prof - double-sword (from AV) - for decent damage, "light blade" property for sneak attack (see below), and "defensive" property
Sneak of Shadows - for the 1/encounter sneak attack capability, since the beast helps with flanking
Beast Protector - MP feat that provides the ranger with an OA if the beast is attacked in melee
Powers:
Circling Strike (at-will)
Predator Strike (at-will)
Twin Strike (human bonus at-will)
Sychronized Strike (encounter)
Paired Predators (encounter)
Partnered Savaging (daily)
Lacerating Maul (daily
Crucial Advice (utility)
Pretty straight-forward suite of beast-oriented powers that allow me to have flanking most of the time. They've generally worked well. Sometimes the ranger deals the big hit; sometimes the beast gets a good shot in. Sometimes they both nail the baddie with dailies.
Twin Strike with a double-sword allows me to wade through minions fairly quickly. Having the beast with me helps with attack bonuses for flanking.
Being a human (bonus trained skill) with the Sneak of Shadows feat (trained in Thievery) has given him seven trained skills, allowing me to use Crucial Advice to great effect with other PCs.
Sometimes, I send the beast off to aid in flanking with other PCs, but most of the time I keep him close to the ranger, keeping that +2 bonus for combat advantage in play much of the time (and allowing the Sneak of Shadows sneak attack capability to come into play in pretty much every encounter).
In close quarters (such as dungeon environments), it can be difficult to move the pair into flank together, but training in both Athletics and Acrobatics has alleviated this a bit, allowing the ranger to run-and-jump/climb/swim over or around adversaries when necessary to get into flank with the beast (who can't do that crazy stuff).
The boar's bonuses when charging (particularly the +2 bonus to speed) have come in handy a few times, allowing the beast to get to a baddie faster than the pair could together. Boar charges first. Ranger follows up on the next round and they pair up in flank for their team powers.
The Beast Protector feat allows the ranger to gain additional attacks when the beast is attacked in melee. If a particular combat really heats up, I move the beast in such a way so as to provoke OAs, allowing the ranger to get a free shot in each time. This strategy generally burns through the pair's hps pretty quickly, but allows for a serious flurry of attacks between them. Using this strategy, a recent 6-round combat encounter resulted in 16 seperate attacks (between the two of them), dealing nearly 120 hps in damage. There were a few misses in this encounter and no crits. Strong damage, despite there being no crits.
Magic Items:
Frost Double-Sword +1
Hide Armor of Fire Resistance +1
Amulet of Physical Resolve +1 (particularly useful for bonuses to saving throws regarding being slowed or immobilized)
Friend's Gift (AV 122) - companion slot item that increases the hps regained when the beast heals - VERY helpful, since the ranger and beast often take a lot of damage between the two of them