Bitcoin is the best cryptocurrency to invest this time because Bitcoin can increase higher before or after so that will be the best chance for you to sell bitcoin at the highest price. But besides of bitcoin, you have another option of the altcoin which can be your second opportunity to make money from cryptocurrency. Right now, ethereum still be the best choice for many people as their investment so you can try to select ethereum too, but you need to analyze before you buy ethereum.
I'd too go for bitcoin and Ethereum. Not only they are trusted and tested, they are also the ones with most improvements and activity on github.
Bitcoin is nearing it's third halving and we have always seen a positive rally after a halving at least on last two. And Ethereum is also going to move to Ethereum 2.0 solving it's inflation and scalability problem.
i would say a lot are still sticking with bitcoin and ethereum. with the upcoming halving, the likelihood for bitcoin to have good gains in the market is very high. and whatever is the trend for bitcoin, most alts will follow and with no exception of course for eth. however, if you want to look for other alts, just make sure they have real development in place and not just pure fud surrounding it