Volibear toplane

I'd say he's pretty viable if you know what you're doing.

He's really strong against other melee champions. In practice he's also really dangerous to some ranged matchups if you can survive long enough to rush Righteous Glory. The trick is abusing his Q and his E after you get hit to get your minion aggro and negate enemy minion aggro for effectively perfect trades, no matter the size of the enemy minion wave. His R also helps in this matter, clearing out the enemy minion wave as you commit to a kill.

His main weakness I've found are Sustain Based tanky champions. Specifically, Cho'Gath. Volibear has more than enough firepower in his base damages alone to 100-0 most squishy champions if you set up on them properly, and thanks to the huge trade advantage he has with his combo and his DPS based damage, it's more than possible to chip down Tanks. However, it costs a lot of mana for Volibear to commit per trade, meaning matchups that can recover quickly in between trades can simply burn through Volibear's mana. Add in tankiness to make them difficult to kill, and you've got a decent recipe for disaster if the opponent recognizes this and plays aggressively.

If you learn to master Volibear however, he's reeeeaaally flexible. He synergizes with multiple Keystones and Runes from every tree and his kit is one that excuses you to build practically anything based on the situation. He can splitpush well with his W and E, you can rush Righteous Glory and Tank items to make him a great secondary engage tool for Teamfights, you can build Juggernaut items to be a stupidly powerful duelist, and some Volibear mains have been playing with more pure offensive items including Rageblade and Nashor's Tooth (only when fed) to push some of the raw power hidden in Volibear's kit.

In summary, pretty viable, you just need to learn him really thoroughly to get the best bang for your buck. If you made this thread because you want to try out Volibear, you're going to be disappointed at first. If you mean to really commit to him however, or if your fundamentals as a Top Laner in general are really strong, you've got a fantastic champion with a lot of potential to try out.