The effects of benzos (benzodiazepines, the name of the class of drugs that valium, xanax, etc. belong to) are somewhat subtle, so yeah, I can imagine that nicotine can temporarily overcome them. However, nicotines duration of like 5 minutes probably makes it negligible.
I wouldn't say that any benzo is 'euphoric', but some are more sedating while others are more anxiolytic. Valium is probably a good balance of the two, while xanax would be more anxiolytic, and temazepam more sedating. I wouldn't recommend trying to get buzzed off them, if that's what you're wondering.