Bitcoin is my favorite CRYPTOCURRENCY, but I would like to make it clear, BITCOIN is not an altcoin. Answering your question, I'll say LITECOIN. Litecoin is the third best cryptocurrency and second best altcoin (next to ethereum), but I've chose litecoin over ethereum because of one thing that made it one of the top altcoins: FAST TRANSACTION SPEED. Litecoin has been very convenient, as per my experience, when it comes on the speed of my transactions with litecoin. Not only that, but it also offer low transaction fee that made me amazed by it. I have already tried sending litecoins from my one wallet with 0.001 LTC transaction fee, and it didn't took so long to arrive at my other wallet (about 3-5 minutes only, I think, before it arrive). That's way faster than bitcoin and ethereum.