What film speed should I use in an older LOMO film camera?

Some quick and dirty tips, for what it's worth:

• Positive (slide) film can be stupendously good especially at ISO 100 and below but demands a very precise exposure for best results. Not best suited to a LOMO, I suspect. I find that 35mm slide film at 100 ISO, properly scanned, is easily as good as the pics output from my Canon 1Ds mark II.
• Negative colour film is more forgiving when it comes to exposure but simply isn't as good as positive. More grain, less vibrant colour. Kodak Ektar 100 comes close, though, but has the downside of needing to be exposed as though it was a positive film!
• Black and white... well, it's black and white. True black and white films can be a true pain to scan, because you cannot use the scanner's built-in dust removal system on it due to the presence of silver grains in the film. And there can be a surprising quantity of dust and scratches on a negative roll straight from the developer. Colour-chemistry (C41) black and white does not have this problem and is easier to get developed commercially - it's actually a colour negative film only without the colours!

When shooting negative film, try to overexpose rather than underexpose. The film can handle it.

I try not to use anything faster than 160 ISO for colour negatives, 100 ISO for colour positives. Simple reason: Fast colour film looks like crap (though I haven't tried the new Portra 400 which a lot of people speak highly of). For black and white this is not so much an issue, C41-chemistry 400 ISO (Kodak BW400 or the Ilford equivalent) is my weapon of choice here due to its ease of scanning. This may or may not be an issue for you. 400 ISO gives a lot of flexibility when shooting I find, and good quality images.

Different colour films render quite differently. Try a few, see which ones you like and which one you do not like.