Pearl is like the more annoying version of Platinum. You have too little of the types you actually need and very few good Pokemon out there. As a result, team building in Pearl is a pain unless you use Infernape or you trade. (The only other Fire-type you have access to is Rapidash. Platinum rectified that by adding Houndoom, Magmortar, and even Flareon.)
Unlike most bird Pokemon you catch really early on in the game, Staraptor is actually extremely worthwhile in the end. (The only other bird that shares this honor is Swellow.) It gets tremendous Attack and Speed, access to Intimidate, and can be a real pain due to moves like Aerial Ace, Brave Bird, and Close Combat. I'd say Staraptor is a must.
When it comes to Water-types, your best bet is either Gyarados or Floatzel. They're both relatively easy to get and they serve their purposes well. If you're really desperate for a Water/Ground type, though, get Whiscash - it's superior to Gastrodon in virtually every way.
Most Bug-types this gen are pretty bad. If you really must have one, let it be Heracross or maybe Vespiquen. Although, Roserade is actually a Pokemon that can hold its own pretty well.
As for other Pokemon you can get later on in the game, not counting starters, all of the following are good/great choices (some of them you need to trade to get but it's worth it):
Alakazam, Machamp, Ambipom, Drifblim, Gengar, Mismagius, Honchkrow, Bronzong (seriously, it's amazing), Spiritomb (very hard to get if you don't have 2 DSs and 2 DPPt games, or you trade), Garchomp, Snorlax, Lucario, Hippowdon, Drapion, Toxicroak, Milotic (if you feel like working for it), Weavile
Platinum has a lot more options for different types, and of the good stuff you can get in D/P, most of them you can't get until very late, or they're incredibly difficult to get (Vespiquen, Spiritomb, Snorlax, Milotic). The key, though, is not to have too many of your Pokemon weak to the same thing - in my run through Diamond, half my team was weak to Earthquake and I suffered heavily at Cynthia's hands because of it. This is why Bronzong and Gengar come in handy because they both have Levitate.
And I didn't list Luxray in there, because Luxray is a terrible terrible Pokemon - even worse than Raichu.
