There are many different types of strike indicators. There are foam pinch ons, yarn, wool, balloons and plastic bubble varieties. Depending our your preferences and the type of knotting you are doing will determine the type of indicator you will want to use.
I have tried them all, some I like much better than others. I carry three types with me when guiding. I have the foam pinch ons for short leash nymphing. I use the Thingamabobber or Air-Lock in the small/medium size (I absolutely refuse to use the large size on principal). I carry (but only occasionally use) the balloons. I will give you a more detailed description below.
My favorite indicator for the past few years has been both the Thingamabobber and the Air-Lock in the medium size. They are sensitive, easily moved on the leader, cast well and are easy to see on the water. They do tend to land hard on the water so for shallow nymphing or slow clear water, other types of indicators work best. Also the Thingamabobbers will kink your leader and both styles don’t work so well on thin leaders, so best used for deep water nymphing.
Pinch on’s work best for short leash nymphing or for fishing in slower clear water. They land softly and you can put multiples of these on your line to help with visibility. The cons for these are that they will not float with heavier nymphs and when removed, they leave a sticky mess on your leader.
I have used the yarn style indicators in my early guiding career. They work well for big or small nymphs and land softly. The cons are that they are tougher to cast and you have to apply a generous amount of floatant to it to make them float well.
Balloons? Not everyone has used them. They are probably the most sensitive off all the indicators. You can inflate them to different sizes, depending on how heavy your flies are. You attach them with a slip knot in your leader. The knot will kink your line badly, so I use this method when I know exactly how deep I will be fishing all day. You don’t want to be adjusting your depth using this method without destroying your leader.
There are a few other styles out there that I have tried here and there but these are my favorites to use while nymph fishing. If you have a favorite of yours that isn’t listed here, we would love to hear about it.