A good lead usually has something to contribute for your team, like hazards, weather setting, putting a Pokemon to sleep, or stopping other leads. I'll review some solid leads.
Swampert is very good, able to set up with Stealth Rock and pivot away with Flip Turn. It also gets moves like Roar to bring more Pokemon into the path of its rocks, and Earthquake to do major damage. It's quite bulky, and has just one weakness.
Swampert @ Leftovers
Ability: Damp
EVs: 252 HP / 4 Def / 252 SpD
IVs: 0 Spe
Sassy Nature
- Stealth Rock
- Flip Turn
- Earthquake
- Ice Beam / Scald / Protect
Ferrothorn is another good lead, with an excellent defensive type combination and access to Spikes and Stealth Rock. It also gets Thunder Wave, Knock Off and Leech Seed to further cripple opponents and Power Whip to take out other leads like Swampert.
Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 80 Def / 176 SpD
Careful Nature
- Spikes / Stealth Rock
- Knock Off
- Leech Seed / Thunder Wave
- Power Whip
Heatran has an amazing typing (watch out for that Ground weakness), and can set up Stealth Rock, Taunt, and Magma Plume. It took a hit now that it's unable to use Toxic, but it's still very good.
Heatran @ Leftovers
Ability: Flash Fire
EVs: 252 HP / 128 SpD / 128 Spe
Calm Nature
- Magma Storm
- Earth Power
- Taunt
- Stealth Rock
Azelf is kind of a weird choice, and probably not as good as some of the other Pokemon on the list. However, it functions as a good suicide lead, with Stealth Rock, Taunt, Explosion, and Knock Off.
Azelf @ Focus Sash
Ability: Levitate
EVs: 252 Atk / 4 SpA / 252 Spe
Jolly Nature
- Stealth Rock
- Taunt
- Explosion
- Knock Off
Froslass isn't good, per se, but it's unconventional and it can work. With moves like Will-O-Wisp, Taunt, and Spikes, plus Destiny Bond, it can catch an opponent by surprise.
Froslass @ Focus Sash
Ability: Cursed Body
EVs: 248 HP / 8 SpA / 252 Spe
Timid Nature
- Spikes
- Taunt
- Destiny Bond / Will-O-Wisp
- Icy Wind
Mew is very versatile, and can have Stealth Rock, Spikes, and Taunt, among other moves, to mess with the opponents.
Mew @ Red Card
Ability: Synchronize
EVs: 252 HP / 4 Atk / 252 Spe
Jolly Nature
- Spikes
- Stealth Rock
- Taunt
- Explosion
Excadrill is excellent. It can set up Stealth Rock, Rapid Spin away hazards from other leads, it's fast, and it can be a potent attacker with Swords Dance, Earthquake, and Iron Head.
Excadrill @ Leftovers
Ability: Mold Breaker
EVs: 4 Atk / 252 SpD / 252 Spe
Jolly Nature
- Earthquake
- Iron Head
- Swords Dance / Rapid Spin
- Stealth Rock
These are some of the more notable leads. If I find any others, I'll add them to the answer. If it's unclear why any of these are leads, I'm happy to elaborate. If you're actually talking about Ubers, as you say in your tags, then Excadrill is the best in this list. Ferrothorn and to a lesser extent Froslass can also do well.
Hope I helped!