What are the most interseting Dwemer Ruins to explore? And how to get into them? I've discovered a few but have no idea how to get in. I know one of them you can't get in till you get to that point in the main quest, but anyways, yeah.

The correct answer is "Blackreach".

Alfstand, Mzinchaft, Avanchinzal would be my 3 picks.

Blackreach, but entering the ruin requires starting the Hermaeous Mora Daedric Quest.
Alftand is a good choice for a Dwemer ruin to explore as a lower level character, most of the opponents are low enough in level that combat is very survivable. The early clearing of Alftand can yield a lot of hard to find enchantments if you remember to bring lots of lockpicks. Alftand can be a trap because the Falmer Boss in the Animonoculary has to be killed to get out of the ruin. The Cathedral section is difficult and letting your opponents fight each other while your character hides is a good option.
Mzinchaleft is another choice for a good ruin to explore, and your character can always just go back if the opponents become too difficult.
Radbthar is not Follower friendly because of numerous traps, but it is a long ruin with decent loot if your character can survive the opponents.
Talking to the Court Wizard Calcelmo in Markarth opens a decent ruin for exploration, but it requires entering Markarth
Sightless Pit is a falmer infested Ruin that is a close second to Blackreach for unique views.

These two for sure - they are huge. Also, in Mzinchaleft you can find a full set of Falmer armor if you're interested.

If you haven't been to Blackreach it may not 'count' as a Dwemer ruin to you, but I won't say why in case you haven't found it.

To get into Blackreach you'll need an Attunement Sphere so if you enter any of the access ruins planning to get in there make sure you have one of those Spheres. Also, when you get into Black Reach make sure you exit via Tower of Mzark or Greal Lift at Alftand so you can get back in directly otherwise you have to trek back through one of the other long ruins.

I did Blackreach without messing with Daedric quests. The MQ led me there. I really enjoyed it.

Uhh, no. You need to at least start Discerning the Transmundane in order to get into Blackreach. Unless you get both item through console.

BLACK..... wait for iiiit......REACH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!......................Period!!

Blackreach is an underused, overrated, undersized underdark wannabe.

Raldbthar, Mzulft and Mzinchaleft are my favorites.

Also Kagrenzel and Sightless Pit are pretty interesting places

Which you pick up at the same time as the MQ points you toward Alftand / Blackreach, conveniently enough. Probably Newaryon picked up "Discerning the Transmundane" without realizing it.

My character's making her way through Alftand right now...just made it as far as the Cathedral before she retreated back to the surface for the night. And possibly to regather her courage. Between the Sounds of Skyrim - The Dungeon mod and the storyline that goes through Alftand, it's a pretty unnerving experience. Nothing like taking a leap of faith off a broken ramp...only to be greeted by a chorus of Falmer screams.

Kagrenzel because of the awesome drop it has, and Blackreach because Blackreach.

I liked Mzulft

I'll admit I have been getting lazy lately, especially with regards to the main quest. I havent explored a ton of Dwemer ruins, but I liked Nchuand-Zel, which is connected to Markarth. I cant really recall the names of the other ones I liked.

Kagrenzel.

Blackreach must be the most beautiful dungeon, but still it can get pretty boring.