Tau will be getting a new codex in the next few months. You can obviously still buy them, but don't start buying/building what currently works in this edition of the codex, because GW tends to turn things on its head (what works before, doesn't now. What didn't work, now works. Although it tends to, it's not always might i add).
Eldar will be getting a new codex at the end of the year. They can be very strong and versatile, but are incredibly hard to use well. Although, they can usually be brought down by spammy lists that other armies can make, which the eldar usually can't do as good as others.
Tyranids can be very strong if you know the right models to use, and if you use cover properly. Again, they are on the rumour train for a new codex but i would realistically guess that we won't be seeing one until 2 years time. Tyranids can be quite tricky to use (besides some peoples comments here on them being point and click, which they are definitely not haha), a fast/mobile opponent can be a big struggle.
On the plus side of each army, Tau are great at shooting and with the new codex might be a bit more mobile than are played now. Eldar dominate the psychic side of a game and can have a unit to counter virtually any other unit. Tyranids can give the largest headache to your opponent by just overwhelming them.