I love every single Tuvie-episode. (Now, that's a big surprise to most here!
Of the three mentioned, "Meld" is probably the best one, writing-wise. It's a great story, it showed us something that we hadn't seen too much before, namely a glimpse of the violence Vulcans are capable of, which is why they have to keep their emotions so much under control. Absolutely stellar acting from both, Tim Russ and guest star Brad Dourif.
"Riddles", in my opinion, is the best acting-wise. Those early scenes, when Tuvok has just awoken from the coma and doesn't speak yet, they are absolutely fantastic. Tim conveys so much with body language and looks. He completely blew me away with this fabulous performance
"Gravity" is very bitter-sweet. It is obvious, that Noss is dear to Tuvok, yet he still remains loyal to his wife and family that are even more unreachable to him than in most other episodes. Once again, many really sweet, subtle touches.
But I also love most of the other Tuvok-episodes. "Innocence", for instance. He is sooo sweet with the kids and it shows us Tuvok as the "family man", how he would probably have interacted with his own kids when they were little.
"Flashback" is another one I like very much despite some flaws in the writing. I particularly how George Takei plays it as if Sulu is seeing something of a young Spock in Tuvie-boy.
"Ex Post Facto" shows Tuvok performing a mind-meld for the first time and is therefore dear to my heart, "Learning Curve" shows him as somebody making mistakes but able to learn from his past mistakes, "Rise" has some good interaction with Neelix, "Alter Ego" sort of starts his relationship with Harry (who turns into something of a semi-regular kal-to partner later on), and so on. Heck, even "Random Thoughts" and "Repression" have their moments (Tuvok's son, for instance, in the latter one). I really like all his episodes, and he also has some really memorable scenes in eps that do not showcase him in particular, "Resistance", for instance. Or "Year of Hell" - the good-bye scene with Janeway.
He may not have been too prominent in the later seasons, but Tim always made use of what he got and did his best with the material he was given.