Best antennas for sites

Sector antennas are a good mix of range and breadth. The dishs are great for long range lnks, but they need to be very accurately pointed, like you mentioned. Omnidirectional antennas are great for serving a broad area, but are more limited in their range, generally. 

Here's a tool you can use to model different options. 
https://link.ui.com/#

These are nice sector antennas if you just want to connect them to a node like a Ubiquiti Rocket M5. 
https://mikrotik.com/product/MTAS-5G-15D120

TP-Link CPE510s are nice, economical choices for 5 GHz sector antennas with built-in nodes. 

A high-end omni like this can be effective if you want to serve all directions. It is hard to find high gain in an omni, though.
https://www.kpperformance.com/5ghz-13-dbi-dual-pol-h-v-omni-1?gclid=Cj0KCQiAwf39BRCCARIsALXWETzFL8dKmeCsTFsIgEhrDr2llEhFlcc-z3RpuV4wXt38TSIZAnqSBl4aAumCEALw_wcB

Sorry if this seems obvious, but: Remember to consider *vertical* beamwidth in evaluating mesh antenna coverage for "omnis" and "sectors". Some are much more limited than you might otherwise think (think looking up and looking down), Also some, like Ubiquiti sectors, have (not necessarily obvious) built-in electrical down-tilt to allow for when determining coverage.

- Don - AA7AU

In general you want to use an as narrow horizontal beamwidth antenna as possible to cover the area or hit the target node.  Yes, this means it must be aligned, but it should be a one time affair.  A little work for a lot of gains (no pun).  This is to not only maximize signal strength on each end, but to exclude as much interference from "the side" as possible.  My rule of thumb is there must be at least one highly directional antenna somewhere unless it's a close-in special event or neighborhood mesh setup or something like that.  This could be a dish or a flat panel style with less than 10deg of horizontal beamwidth. We'll just call them "dishes" below.

The PBE-400-M5 is a good choice, as are the MikroTik LHG dish offerings for endpoints.  For point-to-point connections use only dishes unless the install can't physically support it.  Sector antennas should be used at the central end of point-to-multipoint connections (ie: a sector on a hillside and multiple dishes pointed at it).  There are many Ubiquiti, MikroTik, and 3rd party sector antenna options.  If you have a site needing 360 coverage, the best option are multiple sectors back to back on different frequencies... basically what you see on cell towers. Most sectors are 120deg and their mounts can tilt down to cover the correct area of a town, say from a hilltop, rather than shooting out over the horizon.

The in-between option is something like a Nanostation or SXT which have like a 60deg pattern.  Think of them as short-range mini sectors.  Also, no problem using them for a close-in shot to a distribution sector but don't try to run them for miles due to lower gain.

Omni's should really be avoided as their gain is the lowest and as was pointed out their vertical pattern is quite narrow and not adjustable.  They can be used for close-in distribution, say a short (not hi-rise) downtown building covering a block or two of open space (I'd still use a couple of Nanos/SXTs, personally), or at the center of a special event.  You'd still want directional antennas pointed at it.

Finally, avoid all non-MIMO antennas and radios (Bullets, single-port omni's, and the like) as you're giving up half the performance right out of the gate.

Does AREDN support MIMO functionality?

I was not aware that was included in the software build for AREDN.