The more the merrier
From my experience as a heavy user of rituals in 4e, the more of them you have, the more likely you are to have one that will prove to be useful in a given situation. That said, the rituals I've gotten the most out of are as follows:
GENERAL:
- Make Whole - came across a damaged, unreadable inscription on a
stone. One ritual later, and it was as clear as the day it was
carved. As a generalised repair ritual, it sees a lot of use.
- Tenser's Floating Disc - carries all your stuff, you can ride it
yourself if you don't mind squeezing, also useful for transporting
hazardous materials - e.g. we killed a failed lich, and took his
acid-leaking corpse back to our employer as proof.
- Amanuensis - copies a page of text. Easiest way to copy an
inscription in a language you don't understand, and can also be used
to make copies of secret documents etc.
- Unseen Servant - fear that that box might explode when opened? Send
in your servant instead!
- Water Breathing - lets you breath underwater. Doesn't give you a
swim-speed, so not useful for travel.
- Knock - opens locks. Useful if nobody in the party is good at it.
DEFENSES:
- Eye of Alarm - lets you know if someone is coming to disturb your
slumber.
- Banish Vermin - wards an area against nasty things of size tiny or
smaller. This includes many types of swarm.
- Create Campsite - gives you a nice place to stay, and conceals it
based on your nature check.
- Undead Ward/ Magic Circle - both do much the same thing, except magic
circle keeps any type of creature you specify out.
- Arcane Lock - locks a door, window, etc. for anyone except those
you want to be able to open it.
- Earthen Ramparts - gives you a vantage point and makes it difficult
for others to approach.
- Bolster Object - makes that door etc. extra-tough.
TRAVELLING:
- Summon Winds - makes your boat go faster. Can arguably also be used
to blow away a storm.
- Familiar Mount - if you had a familar, you now have a steed. And if
your familar can fly....
- Phantom Steed - now your whole party can ride... and with a high
enough check, fly.
- Linked Portal - you can teleport to another teleportation circle.
- Tree Stride - step into one tree, step out of another within sight.
- Shadow Bridge - walk across the canyon rather than climbing down and
up it.
INFORMATION:
Last Sight Vision - see the last few rounds of what a creature saw
before it died.
Object Reading - you can see pictures associated with an object's
past. I used it to find out the combination of a puzzle-box that was
too hard to open normally.
Speak with Dead - have a chat with a corpse. You can find out what
they knew in life.
Gravesight - you can see the area around a pre-prepared undead
creature. Use with Undead Servitor.
Inquisitive's Eyes - you can see back in time within a certain area.
Wizard's Sight - lets you summon a scrying-sensor within 20 squares,
even on the other side of a door.
Other rituals that are just a bit of fun to use include:
Fastidiousness - when it needs to not just be clean, but Perma-clean!
Preservation - the text suggests using it to preserve food or
perishable documents, but I used it to preserve a rare flower.
Undead Servitor - raise a humanoid corpse as your servant. More icky than Unseen Servant, so you won't get invited to parties, but it's permanent and it doesn't have to stay near you.
There are more at higher levels, and one of the more recent Dragon articles added a few more useful ones, but these are the ones I had available to me.
From what I have seen of the higher-level rituals, however, many are not necessarily 'better' than the ones found at lower levels, and some are very limited.
The lower-level ones continue to be useful even as your character-level rises, because they do unique things.
