I think many of us started out on Pen or "tube" style mods. I'm rather surprised that nobody has mentioned the Vmax series - still can be found for <= $30 a copy.
These days Dicodes is the "top of the line" for tube mods. TC in a tube? Whats not to like other than the price and form factor?
After amassing a bunch of mods that might work in a pinch but never made it into my daily rotation I've boiled down my mod selections to "use patterns" rather than size, cost or discount.
My "daily driver" (to and from work) is a internal battery "Target Mini". I can draw down 20-30% of the battery just driving to or from work, fits in a pocket rather nicely and I just plug it in when I get to work or get home.
Almost all my single coil builds have migrated to 26650 mods. I can get all day Saturday morning to night (all day) out of them.
I try to match my dual coil
tanks with dual battery mods. (2x18650)
While I'll fire one up every now and then I've pretty much abandoned my tube mods for not being pocket friendly, sucky battery life and no USB charging.
(Disclaimer: I own 1 pico and 3 pico-mega's)