First of all, I do not own any of these currencies, mostly due to the fact that I am a broke college student, and also largely due to my interest in the technology revolution of this space, not in the money.
I take into account several variables when I judge a crypto coin and I will try to list them all. I DO NOT and WILL NOT EVER believe in projects that are a white paper without any code. Another type of project that is clearly not legitimate are ones that are copycats of other coins. I do understand that there are several ideas out there that can be the next big thing, but until I see a working product with at least 60% of its capabilities, then I will believe in it.
I rated the challenges with grades going from 1-4; 1 being easily solved and 4 being very hard to solve. The greater the challenges are does not mean that the coin is not as good.
Well, here is the list and I hope you enjoy it:
1. IOTA

Why is it #1?
It is simple - IOTA is the #1 coin for me because it has the biggest room to disrupt several industries. IOTA has several characteristics that no other coin has, like the transfer of data, no-transaction fees, quantum resistance, and infinite scalability.
Advantages:
- No transaction fees
- Possible decentralization since there are no miners (You confirm two transactions to include your transaction in The Tangle)
- The team behind it (currently a team of 40+ members)
- Infinite scalability (if clustering theory can be implemented)
- Non-Profit Foundation established in Germany
- Partnerships with big companies.
- Growing community
Challenges:
- Deactivation of the Coordinator (4)
- Improvement of the tip selection Algorithm (4)
- Clusters (4) - This more than a challenge, it would be an improvement to the crypto space
- Overgrowth of memory needed to run a full node (3)
- Implementation of smart contracts - If decided that is needed(1)
2. Ethereum

Why is it #2?
Ethereum is really close to #1 on my list. It is the blockchain with highest amount of real use cases today. The tokenization of any asset opens the possibilities to sell anything in the blockchain and this allowed the ICO frenzy this past year. Also, Smart contracts allow the automatization of several tasks and this is one of the few blockchains that has successfully deployed them. Everyday there are more and more useful DAPPS with a better functionality.
Advantages:
- Smart Contracts
- ERC Coins
- Synergy (Each project becomes a multiplier on the existing value of the system)
- Robust Roadmap
- Team behind it
- A lot of companies getting involved with it
- Huge community.
- Sharding (better scaleability and lower fees, coming soon)
Challenges:
- Scalability (currently just 10-30TPS) (3)
- Security (3) - its flexibility derives from Security threats
- Centralization (3) - possibly bigger centralization when it transitions to POS
- Overgrowth of memory needed to run a full node (3)
- Transaction Fees (2)
- Disapprove EIP867/alterable Blockchain (1)
3. Bitcoin

Why is it #3?
The advantage of being the first mover it is not just in mass adoption, but it is also an advantage to have the biggest community behind the project. Several developers work on amazing projects to make this network better. It also has proven that it is a completely safe network so far, making it a perfect store of value.
Advantages:
- Community
- Security
- Adoption
- Upcoming projects (Lightning Network, Rootstock and probable colored coins)
- Robust Network
Challenges:
- Centralization of Mining (4)
- Transaction Fees (3)
- No 1st layer solution for scalability (3)
- Bad actors (2)
4. NANO

Why is it #4?
Transactions cannot get more simple than this. The biggest advantage of NANO is the block lattice - each account has its own Blockchain and this allows for instant transactions without fees. Representatives (chosen with votes from users), resolve problems on the chain. So, what makes this project number 4 in my list is SIMPLICITY!
Advantages:
- Simplicity
- No fees
- Up to 7000 TPS (not tested yet)
- Fair distribution (No ICO or Pre-mining)
Challenges:
- Security (3)
- Centralization of representatives (3)
- Growing Community (2)
- No relevant Partnerships (2)
- Small Team (2)
Honorable Mentions:
Steem and Bitshares: I just started learning about these two projects, and so far they look amazing. I will probably update my list later on and one of these two projects will be a top 4 coin.
Waves: brings a developer friendly approach for smart contracts, I see it as the biggest competitor of Ethereum right now.
I hope you enjoyed this article; later I will write reviews of each of the advantages and disadvantages of each coin. Feel free to comment below.
Special thanks to my girlfriend Paige Cabianca that helped me editing this article!