I agree with jakeru, 18" would be fine for a 'starter pack' but I always use full rods. nothing sucks more than being in the 'zone' and laying down a good bead (especially on alu) and running out of filler. I bought an old standalone wire feeder that I intend to turn into a tig wire feeder someday to get around the running out of rod and cut ends issues.

having 4- 18" sticks of everything under the sun would be something I'd be interested in buying, even today, even at a premium. the odds that I need a full pound of hast W? 0%, the odds I might need to repair a crack in some difficult cast iron? fairly high.
something like
mild:
70s2 (in .035, 1/16th, and 3/32)
70s3
70s6
one of the higher tensile series 80s-,90s-(I know nothing about them, but I'm sure they'd come in handy at some point.)
4130
4340

stainless
308l
309l (in .035, 1/16th, and 3/32)
316l
347 (high corrosion resistance)

alu
1100 (3/32)
4043 (1/16th, 3/32 and 1/8th)
5356 (1/16th and 3/32)
there's another alu alloy rod that jody has mentioned that I'm forgetting that I'd like to try.

nickel
hast w
ERNiCr3-nickel82 (never used it, but it looks like another good compromise rod for welding nickel alloys together and to SS/mild)

other
aluminum bronze
maybe a ti rod and a copper rod?


I'd probably pay a hundred bucks or so for 2-4 rods of each, just to have around, without having to shell out for a lb of each.

I use 2% lanth when I can get it, but almost no place stocks it. I blame this entirely on jody. I have never had an issue with 1.5%, but his shootout convinced me to try the 2% and it does seem to hold up a bit longer than the 1.5%. I've never welded anything over 175a, so I can't comment on 1/8" tungstens. I probably would stick to 3/32" because the need for me to weld that hot would only come up once in a blue moon. including (1) 1/8" tungsten and collets in your pack might not be a bad idea for that reason, though.