This post is going to focus on VHF / UHF antennas that people will use to reach a repeater or node.
As Shorty frequently says, it’s almost hard not to get a good antenna if you buy from a known brand from a good supply house.

The most popular seem to be Comet and Diamond.
Comet and Diamond both have an equivalent antenna but for this post we will use Comet as the example.
Popular Comet models are:

Comet GP-3 – DualBand 146/446MHz – The Diamond X-50 is the equivalent

Gain:
146MHz 4.5dBi
446MHz 7.2dBi
VSWR: 1.5:1 or less
Max Power: 200 watts SSB/100W FM
Length: 5′ 11″
Weight: 2 lbs. 12 oz.
Mounting Mast Diameter: 1 1/2″ – 2 1/4″
Connector: SO-239
Construction: Heavy-duty fiberglass
 

Comet GP-6 – DualBand 146/446MHz – The Diamond X-300 is the equivalent 

Gain & Wave:
146MHz 6.5dBi 5/8 wave x 2
446MHz 9.0dBi 5/8 wave x 5
VSWR: 1.5:1 or less
Max Power: 200 watts SSB/100W FM
Length: 10′ 2″
Weight: 3 lbs. 8 oz.
Connector: SO-239
Construction: Heavy-duty fiberglass, 2 sections
 

Comet GP-9 – Dual-Band 146/446MHz base or repeater antenna – The Diamond X-510 series is the equivalent 

Gain & Wave:
146MHz 8.5dBi 5/8 wave x 3
446MHz 11.9dBi 5/8 wave x 8
VSWR: 1.5:1 or less
Max Power: 200 watts SSB/100W FM
Length: 16′ 9″
Weight: 5 lbs. 11 oz.
Mounting Mast Diameter: 1 1/4″ – 2 1/2″
Connector: SO-239 or Female N-type
Max wind survival: 92MPH
Construction: Heavy-duty fiberglass, 3 sections

NOTE *Although hundreds are in use in repeater systems, it was not designed for mountain-top installation where extremely high winds, ice and other extreme conditions exist. It performs best in low to mid elevation installations where the high gain radiation pattern is most effective. 

Each have their own characteristics which we will cover.