The thing is that unless a poke has a good movepool, it's generally in a low tier. Accelgor really only has Bug Buzz, Giga Drain, Focus Blast, and Sludge Bomb, making it mid-tier at best. Same applies to Cofagrigus. It may be immortal, but it can only use Shadow Ball and Psychic before the Elite Four.
Anyway, here's what I've used:
I still use it because I never box my starters, but it's not good. Leaf Blade and Giga Drain are useful, but it has absolutely no coverage. It learns Strength and Dragon Tail, and that's about it. I don't have time to use moves like Coil and Growth in-game, so it really doesn't matter that it learns them. Still, it's bulky and powerful, so Mid-Tier
I got him in the beginning of the game, and he's pretty cool. Bulky as shit, and decently stong. Great movepool, even though Storm Throw has low PP. Even though this guy is cool, I wish I chose Conkeldurr, as it's pretty much the same poke with a bit more attack and access to Drain Punch. Still, it's a solid High-Tier
I like this guy for his balance. Decent power, decent bulk, and decent speed. Also, not to mention the fact that it's raining for a good part of the game. It's disappointing that he can't learn Earth Power without some fancy breeding, and that Earthquake is only available post-game. It's also very annoying that he can't learn Dive, Waterfall, or Ice Beam, but his Drain Punch makes up for that. High-Tier, maybe Mid
Everyone loves Sigilyph. Seriously, it's amazing. Fantastic movepool, great power, great bulk, great speed. What's not to love? Yeah, I spent about 30 minutes looking for one until I found an Adamant one, and then spent about another 15 minutes looking for another one, but it was so worth it. High-Tier, without a doubt.
Awesome looking, awesome poke. I didn't think Joltik was that weak, especially against the Flying gym. He leveled up quickly and pulverized with Thunder. The movepool is kind of small, but Thunder/Charge Beam/Signal Beam/Sucker Punch is all that you need. High-Tier, but the limited movepool might make some say Mid
I got it early on because I thought I would need it. Its attacks were weak, and it leveled pretty slowly. I didn't need it at all, but I held on to it for a long time. Generally, its defenses were ridiculously low, its attacks weren't powerful enough, and it was just a waste of a team slot. Low-Tier, believe it or not.
Found this guy in Wellspring Cave. HE'S SO GOOD! He leveled up super quickly, soon evolved and learned Earthquake and Rock Slide, making him the ultimate killer. He's actually not that fast outside of a Sandstorm, and mine had really low defense IVs, but he was a monster despite his frailty. He was so good that I didn't think it was that fun to use him. I dropped him in Iccirus city because he was boring to use. High-Tier
I didn't keep him for that long, but he was pretty good. He was powerful, learned Scald quickly, and OHKO'd pretty much everything. I thought it was kind of hard to level him up early on in the game because of pinwheel forest, but he was a beast. I got rid of him only because I though Seismitoad was cooler. High-tier
I didn't really like Cofagrigus. He didn't learn many moves, and he wasn't that powerful. He never died, but killing stuff was a pain, and basic level grinding was hard to do. I quickly dropped him. More useful competitively. Low-tier
I thought this thing was the coolest thing ever, so I used it for a bit. It doesn't learn any good STAB for a while, can't really use Sucker Punch, appears very late in the game, and generally wasn't as cool as I had hoped. Low-tier
Not as terrible as I had thought. He learned powerful STAB pretty early, had nice attack and decent speed and has access to HMs that I needed. He appears all the time in the Winter, which is now, so it's not hard to find him. Nice power, nice speed, nice movepool. High-tier, but if you can't change the clock and if you're not in a winter month, Mid-tier.
Anyway, here's what I've used:

I still use it because I never box my starters, but it's not good. Leaf Blade and Giga Drain are useful, but it has absolutely no coverage. It learns Strength and Dragon Tail, and that's about it. I don't have time to use moves like Coil and Growth in-game, so it really doesn't matter that it learns them. Still, it's bulky and powerful, so Mid-Tier

I got him in the beginning of the game, and he's pretty cool. Bulky as shit, and decently stong. Great movepool, even though Storm Throw has low PP. Even though this guy is cool, I wish I chose Conkeldurr, as it's pretty much the same poke with a bit more attack and access to Drain Punch. Still, it's a solid High-Tier

I like this guy for his balance. Decent power, decent bulk, and decent speed. Also, not to mention the fact that it's raining for a good part of the game. It's disappointing that he can't learn Earth Power without some fancy breeding, and that Earthquake is only available post-game. It's also very annoying that he can't learn Dive, Waterfall, or Ice Beam, but his Drain Punch makes up for that. High-Tier, maybe Mid

Everyone loves Sigilyph. Seriously, it's amazing. Fantastic movepool, great power, great bulk, great speed. What's not to love? Yeah, I spent about 30 minutes looking for one until I found an Adamant one, and then spent about another 15 minutes looking for another one, but it was so worth it. High-Tier, without a doubt.

Awesome looking, awesome poke. I didn't think Joltik was that weak, especially against the Flying gym. He leveled up quickly and pulverized with Thunder. The movepool is kind of small, but Thunder/Charge Beam/Signal Beam/Sucker Punch is all that you need. High-Tier, but the limited movepool might make some say Mid

I got it early on because I thought I would need it. Its attacks were weak, and it leveled pretty slowly. I didn't need it at all, but I held on to it for a long time. Generally, its defenses were ridiculously low, its attacks weren't powerful enough, and it was just a waste of a team slot. Low-Tier, believe it or not.

Found this guy in Wellspring Cave. HE'S SO GOOD! He leveled up super quickly, soon evolved and learned Earthquake and Rock Slide, making him the ultimate killer. He's actually not that fast outside of a Sandstorm, and mine had really low defense IVs, but he was a monster despite his frailty. He was so good that I didn't think it was that fun to use him. I dropped him in Iccirus city because he was boring to use. High-Tier

I didn't keep him for that long, but he was pretty good. He was powerful, learned Scald quickly, and OHKO'd pretty much everything. I thought it was kind of hard to level him up early on in the game because of pinwheel forest, but he was a beast. I got rid of him only because I though Seismitoad was cooler. High-tier

I didn't really like Cofagrigus. He didn't learn many moves, and he wasn't that powerful. He never died, but killing stuff was a pain, and basic level grinding was hard to do. I quickly dropped him. More useful competitively. Low-tier

I thought this thing was the coolest thing ever, so I used it for a bit. It doesn't learn any good STAB for a while, can't really use Sucker Punch, appears very late in the game, and generally wasn't as cool as I had hoped. Low-tier

Not as terrible as I had thought. He learned powerful STAB pretty early, had nice attack and decent speed and has access to HMs that I needed. He appears all the time in the Winter, which is now, so it's not hard to find him. Nice power, nice speed, nice movepool. High-tier, but if you can't change the clock and if you're not in a winter month, Mid-tier.