Pictures of current room or rooms would be helpful.

Mdf baseboards paint beautifully. The downside is they are very flexible so if the walls have waves they will follow them where a pine baseboard would tend to stay straighter and you would caulk where it meets the wall.

Thank you, that is helpful. But, wouldn't what you mentioned as downside actually be an upside? As in, you wouldn't have to caulk and if you didn't caulk there wouldn't be any noticeable gaps between the wall and the baseboard?

You will always have to caulk…

I would replace with real ( i.e. solid wood, primed pine). It looks better, imo. I can’t stand the smell of MDF

I think before advising on what style we need to see your home . I am not one to keep the original if it does not work with my life . BTW Europeans do not leave all the old stuff, that is changeable, to work with how they live in this century I do love the look of the very old like stone walls with very contemporary style but I need to see your home.

Oh gosh, this is not high quality old wood worth salvaging -- this is previous owner half-*ss DIY job with stuff from the local Lowes.

For some background, I'm having new hardwood run in to this area in front of the patio door, and the rest of the hardwood in pretty much the whole of the house sanded and refinished to a different color (probably Provincial or Special Walnut -- something along those lines), then the whole shebang painted. The floor guys will replace the baseboards as part of the job, but I have to buy the baseboards.

If you look at my ideabooks, you'll get an idea of what I'm trying to move my house towards; think mountain cabin with earthy colors, stone and wood.

This whole area will be replaced with wood; job is already scheduled (finally!!):

Other wall in this area; oy that blue is bad but what's worse is the adjacent walls are yellow (!). Haven't picked out new paint yet...will wait until new flooring is in.

This is the adjacent kitchen area; am still in the midst of a kitchen refresh, but here's a shot of the cabinets and new counter and backsplash to give you an idea of color scheme/vibe. I am getting a reclaimed wood island, but it's not in yet.

Baseboards in the hallway; they look like this in the whole house. I think they're horrid and definitely do not go with the overall look I'm moving towards. To boot, he did a poor job installing, lots of gaps and doesn't meet the floor (the bedrooms are way worse...). Anyway, I want to replace with a very simple, plain baseboard all through the house.

Pine. It will look solid and straight.

...

Whatever you put in, replacement baseboards will still likely show gaps at the bottom and top because most floors and walls are not totally flat and the bottom and back: a large, inflexible baseboard will ride on the highest spot. (For example, you can see the variable gap at the bottom of the molding in your last photo.) If you don't want these gaps, you need to add a small, flexible molding at the base which can accommodate these variations at floor level and hide the gaps. Like this:

And you need to caulk along the top edge, which is easier to do with a painted baseboard than a stained one.

Though your present baseboards aren't the best installation, they would look a lot better if they had been properly caulked at the top, and had shoe molding added at the bottom.