I don't really have any for all casters because the lists and roles differ so much from class to class, other than really basic utility stuff like 'someone in the party should have detect magic'. One spell that I think gets overlooked.
Fog Cloud is a really potent 1st level spell. It blocks anyone without blindsight or tremorsense from being able to see - things like Devil's Sight and Truesight don't penetrate it since it causes obscurement, not darkness or invisibility. If the melee characters in a party take blindsight (blind fighting style, skulker, druid forms with blindsight) and casters take a decent selection of 'don't need a target' spells, you can give advantage to you, disadvantage to enemies, and stop their AOOs and specials that require sight while your party remains effective. It covers a huge area - as a 1st level spell it has a 20' radius, and that goes up by 10' per spell level - if you use a 3rd level slot, the 40' radius will usually cover most of a map. While counterspell isn't as overwhelming in 5.5 as it was in 5.0, the fact that it blocks counterspell (since you need to see a target to cast counterspell) can be a huge boon if you have some spells you need to cast, and it's large radius and range mean you can easily cast it outside of counterspell range.
It's one of those spells that you often don't use, but at the right time can completely turn things around. Also it's absolutely hilarious against beholders, since it shuts down their eye beams, except for the area covered by their antimagic eye, which also shuts down their other eye beams.