Can any spell be dispelled unless it specifically says otherwise?

[Yqatuba]

I think so, other than instantaneous spells?

[Azothath]

Yqatuba wrote:

I think so, other than instantaneous spells?

The vast majority of spells can be dispelled and end their duration.

Instantaneous spells can be dispelled/countered depending on the situation (timing).
If a caster has Dispel Magic readied to counterspell OR can cast it as an interrupt than it's possible to counterspell an instantaneous spell.
Lingering metamagic makes Dispel Magic weird and would call for a roll IF the target was in the AoE of the lingering Dispel Magic.
After the casting, yes, those are a done deal and there's nothing to dispel.

Off the top of my head, curses, afflictions(diseases, poisons) usually no but a few have exceptions (rmv curse & dispel magic).
Artifacts are probably another exception.
It's also possible to have a spell that when using normal Dispel Magic isn't numerically dispellable.
Things go a bit awry when dealing with high level spells like Wish and probably your GM will have to make a decision as to partially ending a limited effect or some such thing.

for the most part in most games Dispel Magic is not an effective combat spell and counterspelling is a losing proposition.

[Pizza Lord]

Using dispel magic as the basis, then pretty much, yes. It can dispel any spell unless that spell says otherwise or if it's instantaneous (or if it somehow had an infinitely high caster level that couldn't be beaten except by another incredibly unlikely infinitely high dispeller's caster level).

Dispel Magic wrote:

A dispelled spell ends as if its duration had expired. Some spells, as detailed in their descriptions, can’t be defeated by dispel magic. Dispel magic can dispel (but not counter) spell-like effects just as it does spells. The effect of a spell with an instantaneous duration can’t be dispelled, because the magical effect is already over before the dispel magic can take effect.

Some spells may have continuing or additional effects after their duration expires (which dispelling counts as), and so may still seem to be in effect (like fly, but they're still dispelled.