You should do what you'll have the most fun doing.
That said, there's a couple of basic paths for you to take (assuming, for the moment, a totally Core build - 3.5 PHB, 3.5 DMG, 3.5 MM) that are all fairly strong (and not necessarily exclusive of each other):
Summoner:
This works well at about 3rd+:
You'll need two feats: Spell Focus(Conjouration) and Augment Summoning. That's it. It's okay that you don't have two feats at first level - the one round casting time for a 1 round duration doesn't make much sense anyway. Basically, your tactic is to cast Summon Nature's Ally, and have your summons do the work. Generally, they have better grapple checks than attack and damage (at least for the lower-level SNA spells). But that's fine - it enables the Rogue very, very well (you lose dex to AC vs. everyone not in the grapple - so Sneak Attack!), and the Wizard reasonably well (you lose dex to AC vs. everyone not in the grapple - so usually a reduced touch AC), and the fighter somewhat (you lose dex to AC vs. everyone not in the grapple; when that reduces the overall AC, the Fighter can Power Attack for the difference). It also keeps the opponents off your allies, so there's less patching up for the cleric to do. You'll want to make good friends with the bard - that +X to hit and damage stacks up when you've got 1d4+1 Wolves on the field.
Combat Monkey:
This works at 5th, well at 6th+
You'll need one feat: Natural Spell (which you can't have before 6th - requires Wildshape, which you get at 5th, and you don't have a free feat then without house rules to the contrary). Cast personal buffs, turn into a dinosaur (Deinonychus, specifically at 6th; an Ape is also reasonable). If you can convince someone to cast Tongues on you, you're golden (otherwise, no talking). You will need to speak with your DM about what magic item slots are available on the form you take, though. Or just go non-core and get a lot of those semi-broken Wilding Clasps. Aspect of Nature changes this only slightly; you pick a buff, rather than a specific animal, and you get to keep your equipment (usually).
Melee Combat Rider:
This works fairly well from level 1. You need 1 feat (at least to start): Mounted Combat (for Negate a Hit; the rest of the chain is useful, but not required) and lots of ranks in Ride and Handle Animal. You will want to switch out animal companions at each threshold level upgrade to something else you can convince your DM you can ride effectively, but start with a Riding Dog/Heavy Horse at 1st. Combat Riding general purpose is what you're after.
Animal Handler:
Works well from 1st to about 10th or so, Core.
No feats required, but you want lots of ranks in Handle Animal. Anything that boosts your Animal Companion is a good idea. (Non-core, see if you can convince your DM that Natural Bond will mitigate the level adjustment on advanced animal companions - so that at, say, 4th level, your -3 adjustment Ape gets 4th level Druid bonuses, rather than 1st level Druid bonuses - take it; it's technically RAW legal, but at 4th, the Druid's Ape in Leather Barding with 4th level Druid bonuses will usually take down the DMG NPC Barbarian-4 - unless the Barbarian can keep the Ape from closing). You want to train your animal companions (you'll go through them) for Combat Riding (it's a useful trick list) or hand-pick a set. Unless you get something that has too much Dex to make use of it, you'll want to put some kind of barding on your animal companion - even Leather will help, and there's no ACP in case your DM decides that your mount is not "trained for war". If you can convince your DM that the Combat Riding general purpose qualifies for "trained for war" then (as per the Animal Type description), you can go with heavier barding - but do watch the Max Dex limit. Your spells go primarily to your Animal Companion.
Spellslinger:
The Druid's got a fairly nice spell list - Entangle, Fairie Fire, Obscurring Mist, Warp Wood, Poison (if nobody minds that it is an [Evil] spell), Spike Growth, Spike Stones, and so on can really put the hurt on an opponent's chances of victory. No particular feats required, although any feat that benefits a spellcaster of course benefits this build (empowered Poison: 1d10*1.5 Con damage; Maximized Poison: 10 Con damage - OUCH; and the DC scales with your caster level, not the spell level, so it remains useful until a sizeable portion of your opponents are immune to poison... or you don't want to get into melee range to touch them).
Do note that many of these are compatible with each other.