A couple of quick thoughts prompted by a question from @sofaboy, with a title inspired by a Plone album.
Lamy Safari/Vista/Al-Star (£15/£15/£24 respectively, roughly).
Often recommended as a first fountain pen, and with good reason. The Safari is a tough, ABS plastic pen, the Vista a transparent variant, and the Al-Star an aluminium version.
These are reliable pens that you'll rarely have problems with in terms of hard starts or skipping, and they have an easily interchangeable nib that you can swap for anything from a planner friendly extra fine to an ink gobbling 1.9mm italic.
Readily swappable nibs (most are sold with medium, I have tried that, the 1.1mm italic and the extra fine - all have been smooth, nice writers).
Easily available - I've seen these in Ryman, WH Smiths, Paperchase...
Lamy cartridges are a proprietary format, and international standard cartridges won't fit. (Although Monteverde do now offer a Lamy compatible cartridge - Ryman sell these).
The triangular grip section isn't everyone's cup of tea.
At the time of writing, The Writing Desk (Safari, Vista, Al-Star) have the best price for these pens with the nib of your choice (EF, F, M, B, 1.1mm, 1.5mm, 1.9mm) and a convertor, should you want one. Buying on the high street is possible, but I doubt you'll find anything other than the medium nib on offer.
Schneider Base (£12 rrp)
Where the Lamy range offers a plethora of choice, this more basic pen gives you the option of a "starter" (likely to be very firm, and designed for children just starting to write with a fountain pen that don't know to use less pressure than they would with, say, a ballpoint), left handed, medium or broad nib.
The pen has a remarkably sturdy (and large) clip that will stand up to being used to hold papers together, or being clipped to your jeans pocket with no problem at all. The pen also takes international standard cartridges (long or short) giving you a huge choice of inks.
Takes widely available standard international cartridges.
Left Handed nib if you want one.
Styling may not be everyone's cup of tea.
Limited nib choice.
Lack of a fine, or extra fine nib might limit where you can use it.
Kaweco Classic Sport (£18/£20 rrp (with/without clip)
Whilst I don't actually own one of these (something I'll put right once Tiger Pens start stocking them) I've heard enough people sing their praises to make it worthwhile including the Classic Sport here.
This pen differs from the pens mentioned so far in that it's a pocket pen - these are designed to be small in size when not writing, and then (either by extending, like the Rotring/Parker Esprit, or by posting the cap on the back of the pen, as in this case) larger whilst writing. The Sport measures 105mm long closed, and with the cap posted, 133mm.
The Sport comes in four versions - the Classic is a solid coloured acrylic pen, the Ice a transparent one, the Al Sport aluminium, and the Al-Carbon Sport aluminum with carbon fibre inlay - note that the latter two are far more expensive. There's a decent range of nib choices (Extra Fine, Fine, Medium, Broad and Extra Broad). If you can find a stockist, there are 1.1mm, 1.5mm and 1.9mm italics available too. They take standard international cartridges (short only, because of the small size of the pen).
Takes widely available international cartridges.
Decent range of nib sizes available.
Pocket clip is an "extra" (for £2)
Only takes short cartridges, and you can't carry another in the barrel
You can't use a convertor in this pen
Parker Jotter (£12)
The Jotter fountain pen is surprisingly difficult to get hold of - I bought mine some years ago as an end of line item in Rymans. It's not a pen that will suit everyone - it's thin, and even I don't like writing with it for long periods, but it is reliable, starts after being left for weeks and weeks without writing, and you can add matching pencil and ballpoint (swap the standard refill for a gel one) to it for little financial outlay.
Tiger Pens stock these at £11.95.
The nib comes in medium only, and seems to me to write wet and fairly broad - if you're filling small spaces in pocket planners, you don't want this. However, it's a smooth writer with decent ink capacity in the Quink cartridges (although proprietary, you'll find these easily in the UK).
Inexpensive for a metal bodied pen
Although Quink cartridges are proprietary, they're readily available in the UK, and have a good ink capacity.
One nib size only
Thin body not to everyone's taste
Nib might be a little broad & wet for small spaces and/or poor paper
Some other choices:
I've lumped these together because the nibs are similar - not pens I've used, but my daughter has a P480 that she uses for school. Smooth nibs in fine, medium or broad, and the pens take international standard cartridges.
Waterman Kultur (£12 ish?)
Available on eBay, and from French supermarkets (I got my transparent Kultur at Intermarché in Pont Audemer, for 11 euros). A budget version of the Waterman Phileas, more or less, with a nice smooth, fairly wet nib. They take international cartridges.
Noodler's Ahab / Konrad (£20 ish including shipping).
Not sold in the UK, but US sellers like Goulet Pens will ship to the UK for a total cost that's around £20, depending on exchange rate. Slightly more finicky than the other pens in this article (usual advice is to disassemble and clean before using - don't worry, this is easy, and instructions come with the pen), but with a fun flexible nib that writes nicely. In normal use, you get a fine, fairly wet line, but with more pressure the nib will flex to give a degree of line variation - this makes them quite fun to sketch and doodle with. These take bottled ink only.
I've tried to keep these options at £20 or less, but if you're spending more, consider;
TWSBI Diamond 540 (£42)
A piston filling, transparent pen with fantastic build quality. Takes bottled ink only, and can be disassembled, tweaked &c. TWSBI even give you the little wrench required, and "how to" videos to help. For an extra £5, you can get them in coloured transparent plastic.
I've omitted vintage pens entirely - although there are some that can be had for reasonable prices, and would do good work as daily writers, choosing one requires either a bit of knowledge, or luck. I've no time to write up even my limited knowledge, and the luck I can't impart to you by writing about it!
Pens with proprietary cartridges will limit you to that brand of ink unless you use a convertor - however, Lamy and Parker both make a decent blue and black (I'm rather partial to Lamy Purple as well). In international cartridges, Waterman Florida/Serenity blue, Blue/Black, and virtually any Diamine will serve you well. If you write on a lot of poor quality paper, Pelikan Blue/Black works nicely, although it can feel a little "dry" in some pens. Diamine offer excellent mixed packs of cartridges that will allow you to try a large array of colours.
Bottled ink offers a huge choice, and you're only limited by the retailer's stock. Diamine do offer small (30ml) bottles, so you don't have to commit to a full 80ml bottle! None of the cartridge filling pens listed come with a convertor (and the body of the the Kaweco Sport is too short to take one), so if you want to use bottled ink, you'll need to budget £3 - £4 for the convertor as well.
Personally, I like at least two notetaking pens, with distinguishable colours in them - I tend to use a blue, blue/black or black (of varying degree of sobriety, from the cheery Diamine Turquoise, to the all business Pelikan Blue/Black, or Herbin Perle Noire) with something brighter, Diamine Monaco Red, Imperial Purple or Red Dragon are all good choices for the latter.
The paper you write on will affect the ink, and nib you choose. In general, the Clairefontaine paper in Rhodia and Quo Vadis products likes most inks, and I've had good luck with the less expensive Black N' Red, and "Pukka Pad" products. Moleskine has somewhat more variable paper, and large, wet nibs, and some inks will feather and bleed on it. If you insist on using Moleskine, I find fine and extra fine nibs, and dry-ish inks (Pelikan Blue/Black) work best. Inkyjournal.com has rated many inks for Moleskine friendliness, so if you want to pay a premium for poor paper, you can do so with confidence :)