DASH, ETH. Monero all are good in one way or the other but i must say that Bitcoin will remain to be on top of all these as BU will never be succeeded and even if does people will not accept it.
But under what circumstances? As things stand now bitcoin TX fees are much worse than any other crypto, and it's been causing a steady stream of bad news in the press, in merchant announcements and service cutbacks or restrictions and so on. I have no confidence in BU's approach or competence, and no confidence in the ability of Core to reach 95% consensus for Segwit. The UASF is a longshot and a bad precedent IMO.
The alts market has basically doubled in size since the Winkelvoss ETF rejection. All it would take is another draft of bad news - say, China taking advantage of bitcoin's lack of privacy features to clamp down, or another major bitcoin heist/hack, or even some major good news for Ethereum like the launch of the Raiden network or a major smart contract development - and we could see bitcoin drop below 50% and be in a fight with Ethereum for #1. That's the fast scenario. The slower scenario of Bitcoin continuing to strangle while alts progress on many fronts just drags things out for months longer, until enough BTC diehards accept the inevitable and the stampede starts.