Druid 20 is one of the strongest characters in the game. Wis > Con > Int > Dex > Cha > Str.

Feat choices in order of power (and where to find each one):
1st (and bonus at 1st via human, flaws, etc.): Greenbound Summoning (LEoF), Companion Spellbond (PH2), Wild Cohort, Natural Bond (CV), Spell Focus: Conjuration + Augment Summoning, Combat Reflexes, Extend Spell. Companion Spellbond is high on the list due to the long duration buffs you can share, which will end if it gets out of range (5 ft. normally, 30 ft. with that feat).

3rd: Natural Bond (CV), this is not optional unless you took it at 1st, in which case get Item Familiar (UA). Get a 'level -3' companion at 4th level such as a Fleshraker dinosaur (MM3) or a Dire Eagle (RoS, text > table) and the 'level +3' for Natural Bond will make up for that drawback and you can count your full Druid level for its benefits. If you got Wild Cohort, at 7th level you dismiss both your wild cohort and your animal companion and get a new companion which is both, thus gaining the benefits of both progressions.

6th: Natural Spell, this is not optional. It should be self-explanatory.

9th+: Persistent Spell*, Quicken Spell, pretty much anything you want. *If you choose an Illumian (RoD) for your race with the Naenhoon sigil and apply quite a few Unguent of Timelessness (DMG) to eight times as many Bone Talisman (turning) spell foci. Cast that spell during downtime at 365 times the normal duration (doubled again if Extended) and during adventuring days expend those turning uses to add Persistent Spell to Bite of the Weretiger (SpC). At 9th level and extended each Bone Talisman should last over 45 days. With Wis 16 you'll have five 2nd and four 3rd level spells per day, so with two Lesser Metamagic Rods of Extend and Extending them in the 3rd level slots normally that's nine Bone Talismans per day without using up even higher level spell slots. Number each bone and rotate them so that you always use the oldest, shortest-remaining-duration bones first. Ideally you'll have at least thirty days of downtime, which will give you enough bone talismans for 45 days worth of persisting one spell per day, using the oldest ones first so that the last six you use will still be on the last day of their duration. Just spending two 2nd level spells per day of adventuring to replenish your supply will extend that to 67 days worth of bone talismans, and on the last day of that you'll use the two you cast that same day. Definitely get Companion Spellbond if you do this so that you can share that Persistent Bite of the Weretiger with your animal companion, though it will end if you get further than 30 ft. away from each other.


Animal Companion: Wolf at 1st-3rd level (Wolf at 1st-6th as a Wild Cohort), Fleshraker Dinosaur (MM3) or Dire Eagle (RoS) at 4th+. Use the Handle Animal skill to give each of those the Warbeast template (MM2). A Dire Eagle should get Combat Reflexes and probably the Mage Slayer line of feats (CA). A Fleshraker should get Ability Focus: Poison (MM), Virulent Poison (SS), Power Attack, and Leap Attack (CV).


Items:
A wooden tower shield will serve you well at levels 1-4, don't worry about the nonproficiency penalty just cast Enrage Animal and other spells that don't require an attack roll.
Wilding Clasps (MIC) which allow items to continue to function when wild shaped.
Healing Belt (MIC) as early as possible, to be replaced when you can afford...
Monk's Belt, which gives you a Monk Unarmed Strike which can be used in animal forms and the Monk AC bonus including adding your Wisdom bonus to AC. It should be Wilding Clasped, and it 'continues to function' when you wild shape; one function of a belt is to hold other items, so nothing you store on/in your belt will need its own Wilding Clasp.
Armor/Mantle/Ring of the Beast (CC) great bonuses, Wild Armor melds into your form when you wild shape and you're not considered to be wearing armor so you can still benefit from the Monk's Belt with it. None of these need a Wilding Clasp, that effect is already built in.
Lesser Metamagic Rods of Extend, get at least three. Store them on your belt.
One Standard Metamagic Rod of Extend and one 6th level Pearl of Power at level 14+, to be explained later.
Circlet of Rapid Casting (MIC), with an Enhancement bonus to Wisdom added as per MIC page 233-234. While you're on those pages, see what other bonuses you can add to your existing items, particularly your Mantle and Ring of the Beast.
A truckload of Unguent of Timelessness (at least 51), and a lot of free bones, see above. Decorate your belt with them.
Standard Metamagic Rod of Empower at level 11+, to be explained later.
Wand of Lesser Vigor (SpC) to heal up between encounters.


Spells, presume a Lesser Metamagic Rod of Extend should be used with any hour/level buffs of 3rd level or lower, all buffs should be shared with your animal companion whenever possible:
1st: Longstrider, Snowsight* (FB), Produce Flame*, Summon Nature's Ally 1*, Enrage Animal* (SpC)
Snowsight is in Frostburn, see the 2nd level spells below. Produce Flame should be used to add fire damage to your natural attacks when wild shaped, use the Circlet of Rapid Casting and a Lesser Rod of Extend to cast it and full attack. Summon Nature's Ally 1 can get a Greenbound Dire Rat which can use its Wall of Thorns spell-like ability as soon as it appears, then spam Entangle for the remainder of its duration. With a Ring of the Beast you can instead get a Greenbound Dire Bat which can do the same thing but is also a capable melee combatant. Enrage Animal should be cast on your animal companion in the lower levels to make it even better at fighting when you have nothing better to do than concentrate on a spell the whole fight, later on you can get the Swift Concentration skill trick (CS) and start using it again.

2nd: Creeping Cold* (SpC), Bone Talisman, Obscuring Snow* (FB), Heat/Chill Metal, buffs, etc.
Creeping Cold can be cast with a Lesser Rod of Extend to deal 21d6 damage over six rounds at 3rd level. Obscuring Snow combined with Snowsight creates a battlefield environment that opponents cannot see in but your party can. Get your other party members who want Snowsight cast on them each day buy you a 1st level Pearl of Power and 1/3 of a Lesser Rod of Extend, so it costs you no additional spell slots to put an Extended Snowsight on each of them. Heat Metal and Chill Metal cast on an opponent's metal weapon should cause it to be dropped, as per Gandalf pwning Aragorn in Fangorn Forest.

3rd: Greater Magic Fang (+1 to everything, on your animal companion), Sleet Storm, buffs, etc.

4th: Summon Nature's Ally 4 for a Unicorn to heal people better than your 4th level spell could, and it has Magic Circle against Evil in case anyone is charmed or dominated or if there's extraplanar enemies. Use the 3rd level summon with a Ring of the Beast. Greater Resistance (SpC), Ice Storm, buffs, etc.

5th: Animal Growth on your summoned lions and animal companion, you can choose not to make them greenbound so this will work on them. Entirely optional. Call Avalanche in Frostburn, buffs, etc.

6th: Fire Seeds (berry bombs) cast with the Rod of Empower, give the berries all the same command word and put them in a pouch; drop it at the center of four opponents and deal hundreds of damage to each of them. Superior Resistance* (SpC), Energy Immunity* (SpC), buffs, etc.
By the time you get higher level spells you should probably know what to look for.

Once you get a few 6th level spells per day, you should have a Rod of Extend and a 6th level Pearl of Power. Every other day prepare Energy Immunity twice and recovering one with the pearl use the rod to cast it three times. Pick a different energy type for each one, they'll all last 48 hours thus doing this every other day. On the days you don't do that, prepare Energy Immunity once and Superior Resistance once, and use the rod to cast each of those and Energy Immunity again using the pearl. Pick the last two energy types, all of those will last 48 hours. That makes you continually immune to all five energy types (Acid, Cold, Electricity, Fire, Sonic) and gives you a +6 Enhancement bonus to all your saving throws.