I think Legendaries may be not allowed in gyms. But I am not sure so correct me if I am wrong. But to answer your Question yes having a Level 30+ Tyranitar with B/CR and B/SE will be very usefull also RT/SE Golem will help alot. Plus as a Secondary dark attacker... a double dark gengar would be fun.
Best buddy before legendary
Larvitar family is the best walking buddy now?
Answers
Larvitar, machop and the same old dratini. Geodude also have my attention, since rock moves golem will be the ultimate counter for at least 4 legendaries, moltres, ho-oh, zapdos and articuno, and also great against lugia, and he is a 1 km buddy. If you dont have at least 2 great golems and have trouble finding geodude candies it might be a good buddy.
Due to the inclusion of Rare Candy, it's less effective to buddy the "best" Pokemon, as they usually have high distances. Your walking buddy should be a strong Pokemon, with low distance.
I'd say your best choice is Golem.
He's the highest DPS available for Rock type moves, and grants a candy for every 1KM.
I am walking Mareep, I have ever found only two, and after some 60 km I still have only 22 candies :/
I still need 103 and then the amount to get it to level 30, so probably 200 candies total, times 5 km is 1000 km!! Of course hoping to catch/hatch something instead. Not hoping for an electric event though, which would render the hard work useless.
I think he will do fine and I hope he will still be a rare sight (currently still never shown off in gyms here) when I finally get one. But some multiple bar charge moves would have been nice. I have Zap Cannon on Porygon2 and when prestiging against him, it was so slow that you could dodge and wait seconds for the move to complete...
And I hatched a Mareep today from a 10 km egg! Resulting in +30 candies at once!
You buddy to simply get candy...there is no other purpose currently...
I'd say buddy whatever you need and Larvitar is what I buddy and the safest/most useful for now I think...
Golem is nice, but when you have 500+ and had 1000+ candies (and using them just for evolve Exp), it's faster to just catch them since they are super common in the desert...