Tapu Bulu is generally outclassed by Rillaboom. Rillaboom has fewer crippling weaknesses, plus higher Speed and a greater movepool.
Tapu Lele is an excellent special attacker, and with moves like Psyshock, Moonblast and Thunderbolt, plus the help of Choice Specs or a Scarf, it can tear through teams.
Tapu Koko is also an excellent special attacker, plus it's a brilliant pivoter with its Speed and STAB Volt Switch. It also gets Roost for reliable recovery. It is pretty frail, however.
Tapu Fini is the best defensive Tapu, with 115/130 defenses. With a few Calm Minds, it gets even bulkier, and can also KO its fair share of opponents. It lacks good recovery though, with Draining Kiss being its best option. It's tremendously useful in walling an opponent.
It obviously depends on the rest of your team, but they're all pretty good at what they do (except Bulu, that's outclassed by Rillaboom).
Hope I helped!