The frixion pens are the main ones I use, I like the fact that they're more like felt pens and there's no bleed through the paper. Likewise the frixion colour come in a good variety of colours so I can match them to my planner themes each week. I got them from Amazon, the only downside is they take ages to get here as they're imported.
Poppy liked helping me give the frixion ball pens a use. These again come in a variety of colours and have no bleed. They are however a little pricey for what they are. I prefer these for my work planner as they feel a bit too sensible for a fun pen.
My staedler fine liners are great, lovely and bright but there is a slight bleed through the page. My writing also looks tiny in them in my A5 where there are bigger pages and spaces to fill. These are ideal if you need to maximise space and aren't too bothered about covering the bleed with stickers. Personally, I prefer these for note taking.
The gel pens are one of my favourites and I own an insane amount of them, but on the whole I prefer these for my work planner or making notes. There is no bleed through and the colours are nice and bright. If you've got a good pen they're lovely and smooth to write with, but I've going there tends to be inconsistencies between pens and some are a bit scratchy. I don't want this going on in my planner.
Fountain pens are my absolute favourite thing to write with ever. I've been using them since I was about ten years old and I love the way you can get different nibs and inks, not to mention styles and colours. There's just something that feels really grown up writing in a fountain pen I feel.
The only time I will absolutely not use fountain pen is when I am writing in my planner. Whilst the writing looks gorgeous, the bleed I get from them is really heavy. I learnt this the hard way earlier this year and spent the following week strategically placing stickers, tape and sticky notes over blotches of ink.
How about you, what do you like to write with in your planner?