1) Any main differences within them and what's their best strength against each other?
2) As i know block time for BTC is averaging at 10 minute and recently if you sent not enough fees your transaction takes really long time.
I remember i read an article about ETH transaction just take within 15 seconds and the opposite will receive, is that true? I only uses BTC to transact.
How about Dash?
Thanks!
I think when you talk about a coin, you can't just focus on it's technology features. You must always look at it's community, it's dev team and their long term goals. I like dash but their goal is to become a payment method, and they just don't have enough liquidity of support from the market in order to accomplish that. You can't become a payment method without enough liquidity, because you will just be to volatile. Who wants to pay with dash, even though it was fast, and it had low fees, and then just see it's price go up and see that they just "lost money" on that purchase.
This is where bitcoin excels, if you compare it to any other coins. Bitcoins want's to become a valuable asset first, and then given it's technology, will probably become a payment method or a global currency as well. Things need to be done in that order, and not the other way around.
As for ether, their goal is to act as a platform to allow investments for new projects. They could become a payment method, but I don't think they want that.
So for your first question, I would say that bitcoin and ether have a clear purpose, and are following it in the right direction. Dash also has a clear purpose, but are not pursuing it correctly (I would say that is the same problem with bcash for example).
As for technology, yes, dash allow faster transactions now, and has lowers fees etc, but that doesn't make it better than bitcoin or ethereum. BTC and Ether, will solve those issues in the long term (with lightning network and raiden network respectively), so I would invest on them, and not on dash.
All coins that want to be act as payment methods are overpriced right now in my opinion, expect for LTC.