Do you want a lot of opponents with your K-K? No you don't! The more opponents, the higher the risk such a top hand is going to loose its value at the Flop. So it's crucial to raise such hands right from the beginning and even re-raise other player's raises. You want to play against other high cards or lower pairs to stand the best chances. With just a few opponents at the Flop this is a much easier job. It's ideal the have the Overpair at the Flop, which means the pair in your hand is higher than the one on the board.
But be careful when the card on the board is higher than your own pair! That means another player could possibly have a higher pair. However, the Preflop will already reveal: The more players raise and re-raise here, the more likely hands like A-K or A-Q are in the game. You have to keep that in mind, especially with hands like J-J and Q-Q to avoid rude awakening at the Flop.
Medium pairs like 9-9 or T-T
Medium pairs are difficult to play. Battles against A-K for instance are truly no safe bets! It's more than a remote likelihood to get the Overpair with a 9-9. These pairs yet still may be interesting if the Flop only consists of low cards, because then all those people playing with an A-Q for instance don't have their desired Flop and all over a sudden the 9-9 is the superior pair.
High cards like A-K or A-Q
The favoured Flop is a top pair. Two high cards also mean you still have a good Kicker besides the high pair. The Kicker may decide a game if two players play with the same pair. That's why A-K is a popular hand to raise: you can decrease the number of your opponents and hope to make the top pair in order to put even more pressure on your remaining opponents.
Notional hands like 3-3 or 8-9 suited
These hands really can become strong at the Flop, though most of the time they'll be a disappointment. 3-3 only gets stronger if the triple turns up. The ideal Flop for 8-9 suited (in the same colour) would be two pairs, Flush or Straight or even an uncompleted hand that still stands some chances.
These hands just have one thing in mind: to create a monster hand. That means you should invest just a minimum into these starting hands, but you can only achieve that for sure in a late position, of course. If you have the monster hand, however, it would be ideal to have a lot of opponents to get as much money into the pot as possible.
