There have been many threads on the topic. Many recommend the Ammelite, Lincoln and OTC pistol grip guns. Others recommend John Deere's as well. I ended up getting the OTC on Amazon
https://www.amazon.com/dp/B00EVPLRRU/?tag=atomicindus08-20
I have this one for greasing the tractor. Works well. Used to be USA made not so sure anymore.
https://www.amazon.com/gp/aw/d/B009K530KS/ref=psdcmw_15707871_t3_B009K52WNY
A good grease tip makes a big difference too. I would recommend a Lock-N-Lube tip. I got the long one and 95% of the time it fits in tight places.
I have the short one and really like it too. I have been thinking of getting the long one but could use more info on how it works and if better.