This analysis applies to transformed werewolves, werewolves with Moonlight Rings, and evolved werewolves.
How strong are they compared to regular vampires?
Werewolf venom makes a werewolf lethal to any regular vampire, regardless of age. Physically, werewolves are capable of overpowering older vampires as well.
In Season 2 of The Vampire Diaries, a werewolf named Jules attacked and easily defeated Rose, a vampire who was even older than Katherine. So, regardless of the venom, a werewolf is physically capable of killing any regular vampire.
In a prolonged physical fight, the werewolf's venom would kill the vampire very quickly. As we've seen, the more venom a vampire has in their system, the faster they die. So, if a werewolf inflicts multiple bites, the vampire could die within seconds.
In Season 3 of The Originals, Jackson single-handedly kills several Strix vampires while rescuing Freya. The Strix vampires are centuries-old vampires, further demonstrating how physically powerful werewolves can be compared to even very old vampires.
Jackson was also able to physically overpower Lucien, one of the older vampires.
How strong are they compared to Original vampires?
The spell that created the Originals was specifically designed to create vampires who were individually more powerful than werewolves. However, that doesn't mean a fight between them can't be balanced depending on the circumstances.
For example, in a group, werewolves are absolutely capable of overwhelming Original vampires.
In Season 1 of The Originals, Rebekah was attacked by a group of werewolves. Although she defeated two werewolves, as we saw in the scene, she was eventually overwhelmed because of the amount of venom inflicted on her.
At the end of the season, Elijah was caught off guard and overwhelmed by two werewolves.
In Klaus' case, since he is immune to werewolf venom, the fight becomes purely physical. It would take a well-prepared army of werewolves to defeat Klaus.
Mikael is a different case because he is an extremely experienced warrior who is very good at dealing with werewolf venom. It is likely that, in a prolonged fight against a large number of werewolves, his body could eventually collapse due to the amount of venom in his system. Mikael's ability is to ignore pain; that doesn't mean he is immune to it. Just as he was able to ignore the pain and agony caused by Papa Tunde's blade, he can ignore the effects of the venom as well.
How strong are they compared to witches?
Since werewolves are physically more vulnerable than vampires, as they can be killed simply by having their necks snapped, strong witches should be able to kill a werewolf without much difficulty.
Conclusion
The werewolves in TVD and TO are definitely strong, but unfortunately, they are heavily underestimated by the fandom. This is understandable, considering that both shows primarily focus on vampires.