More concerned with rank than lane? Try a one-trick champion rather than a specialist champion.
DISCLAIMER: I'm Gold V and this is geared towards Bronze/Silver players who wanna climb easily.
Specialized champions mentioned in another post are champions that are able to fulfill multiple roles/maintain presence in multiple lanes by providing zone pressure.
However, I have a different suggestion.
One phrase: One-trick.
After primarily playing a single champion for months, and learning their kit in and out, with every interaction with every other champ in the game (except volibear because no one uses him), you'll start to become a "one-trick".
This is different from maining a champion. For example: I used to main Xerath, but also played as Caitlyn, Tristana, Lux, and a slew of other champions.
Then I began to play exclusively Vayne unless I really needed to fall back on Caitlyn as a counterpick. That is the spirit of one-tricking, and while it limits you, it's good for rank climbing up to Gold/Plat. I just wanted season skin rewards so I got to Gold and just let it sit.
Different champions are good at being one-tricked because they become so damn good once you get really good at them. The classic one-trick example is Nidalee, with an exceptionally mechanically "overloaded" kit. Yet, each of her abilities on their own are extremely weak. The combination of utilizing each of them effectively is what lets Nidalee one-tricks become so good at Nidalee.
Some champions that become extremely terrifying if you become extremely good at one-tricking them are because they have mechanically intense but also rewarding kits, like Azir, who is nerfed repeatedly for his high elo performances, while other champions become exceptionally powerful in other aspects once you learn the inner workings of a kit to catch people off guard.
Usually, you trade a lot of time and investment into a champion that has the potential to outshine every other champion in the game in a given area. For example, a perfect Vayne player might be able to carry late game every game, but it's so difficult to do that that no one really manages to achieve that level until maybe Challenger. Otherwise, it's simply a kit that requires deep knowledge to counterplay, like Gangplank's barrel placements and combos.
Champions that are mechanically intensive but rewarding:
A special section for two sided champions for having essentially "two kits":
While Gnar is mechanically intensive in managing his Rage bar, the other three are mechanically intensive in knowing when to switch forms to obtain a given benefit/obtain a given ability that would be useful for a given situation, and having to make that decision in cases under 0.1 seconds can be very hard to do without a lot of practice.
Other mechanically intensive but rewarding champions:
(Bard is more reliant on the in depth-ness of his ult, and how much it can impact a game) 
(This champion is busted even if you don't know how to perfectly play her, but she just gets SO MUCH stronger when you do. Meet Irelia.) 
(Kaisa is similar to Irelia.) 
(Another Irelia/Kaisa but recent nerfs toned her down a little) 
As you can see, there are quite a lot of mechanically intensive but rewarding champions.
Other champions that benefit through being able to use "hidden" interactions in their kit that are easily overlooked/hard to consistently play around in a high stakes fight that are still hard to utilize without knowing a kit in and out:
These are usually SIGNIFICANT things that people forget.
: Egg, Walls, and a teamfight ult make this champion both hard to play but also extremely powerful due to the surprise factor.
: Everyone forgets her shroud has "True Stealth" now, so dropping a control ward isn't going to help, and Akali's E allows her to dash to a hit target, even from across the map.
: His ultimate does increasing damage based off of how much health he lost, so don't continuously fight him hoping he'll pop without taking into account his ult's position.
: His ult has a small "Splash box" at the location Fizz aims his ult at. THIS IS IMPORTANT WHENEVER YOU ASK WHY "Fizz's fish's hitbox is bugged". It's not. When the fish lands at the location Fizz sends it at, it will actually "bounce" to the closest target within a very small radius. Watch out for this shit, and remember it if you're playing as Fizz.
: His barrel game.
: Heimerdinger's W will instantly proc all turrets' "big beam" if it lands on you.
: Kha'zix does increased damage to targets with no one else near them.
: Rakan can use his R before using his W to guarantee a knockup on W if he passes through you while using W.
: Her E will stop channels, and some continuous spells like Rammus's Powerball.
: If you use an item's active or a summoner spell, Zoe will be able to use it as long as she's not holding another one and is able to reach that other spell. Be. Careful.
Champions that are able to have high impact on all lanes simultaneously despite not appearing to be able to at face value once you get good at a specific aspect of them:
E isn't for getting to lane faster, it's to allow you to roam instantly to top OR bottom.
R. Bard's ultimate is good while Bard is roaming, able to slow down both Mid/Bot at the same time/turn the tide of a battle.
After Level 6, she gains permanent stealth.
His ability to apply hidden pressure due to the nature of his R.
this entire champion
Sheer movement speed provides massive pressure on all lanes at all times.
While it doesn't look like she's that scary in terms of damage, after level 6 she can practically force any lane into a skirmish with her R as long as the enemy overextends the tiniest fraction.
Through his E, he's able to roam extremely quickly, and get out extremely easily.
Through random Redemption shard drops, she can actually assist another lane (usually bot) by providing a boatload of healing to the characters in that lane, or provide zoning pressure, or provide waveclear assistance.
If I don't have a character on this list, that means that I probably haven't tried them out. Feel free to share hidden tidbits of knowledge and stuff!
If you want to try out a character, just search "How do i play (X champ)" and watch some videos.